Project Blue Book Special Report No. 14 (Analysis of Reports of Unidentified Aerial Objec…

----~.~ ..:_:._~,. / FORM 2250 Approved fo..r: Release 2026 Under Section 1842 of the National Defen~5.e ,..,_ _.,-<,y-; ~ -- --<-4H~ 6 64 1 • Authorizati6n Ac-e' for Fiscal Year 2024 ~ 5'f.J _: ~- _,.,/ 1 (PLACE FORM qgo HERE) 0 FFI C/IAL J RECORD The, Agenc. in accc. additior, Records .,.---------- RETURN IMMEDIATELY AFTER USE TO THE CIA ARCHIVES AND RECORDS CENTER 6-64 FORM 2250 (PLACE FORM ~90 HERE} OFFICIAL J RECORD COPY WARNING The attached document (s) must be safeguarded. It is the Agency's Official Historical Record and must be preserved in accordance with the Federal Records Act of 1950. For additional information, call the Chief, CIA Archives and Records Center, extension 2468. RETURN IMMEDIATELY AFTER USE TO THE CIA ARCHIVES AND RECORDS CENTER SJ -:/). ,1/i . . . r ..~.. ,=,. _._,_- . : ~ ·, ••,;-.,-·, C / , ' \ .- _.," _ :.--- _J: -~,,,, , -,_1: ... ~ - ~- ·~f·;•";,_,, ;: ~;..::;s.-..:.--:;_1-:,:,,•-'l(<..., --... ~·-....... --..._,,,!,.,,,,.,._ I -~ ~- I /) ,._;· C'.'.:'.: ?'e·--·('· ·~,-- ~~lt!L j ,.,.. # ·, • ·•21u,c< .."""- 1 d'" ; I 'PROJECT BLLI ' B • ti?0 1---7 I '/~ I SPECIAL REPORT N0.14 II I (ANALYSIS OF REPORTS OF UNIDENTIFIED AERIAL OB1ECTS) I I PROJECT NO. 10073 I I 5 MAY 1955 I \ I I I .I AIR TECHNICAL INTELLIGENCE CENTER I WRIGHT-PATTERSON AIR FORCE BASE I OHIO I I Copy No. 35 --=---- ,, l PROJECT BLUE BOOK \I SPECIAL REPORT NO. 14 ii ' (ANALYSIS OF REPORTS OF UNIDENTIFIED AERIAL OBJECTS) il 1 I I I PROJECT NO. 10073 I :1 5 MAY 1955 I 1 11 ! ii FOR OffIC!Al USt ONlY (AFR 190-16) I :I !1 I I AIR TECHNICAL INTELLIGENCE GENTER I WRIGHT-PATTERSON AIR FORCE BASE o:mo I I No copyright materiel ls contained In this publication, _I I I TABLE OF CONTENTS I SUMMARY. vii I INTRODUCTION ORIGIN AND NATURE OF DATA 3 I REDUCTION OF DATA TO MECHANIZED COMPUTATION FORM 4 I Questionnaire . 4 Coding System and Work Sheet 6 Identification of Working Papers. 7 Evaluation of Individual Reports 10 I ANALYSIS OF THE DATA. 14 I Frequency and Percentage Distributions by Characteristics 14 Graphical Presentation . 16 Advanced Study of the Data 16 Position of the Sun Relative to the Observer 16 Statistical Chi Square Test 60 The "Flying Saucer" Model 76 I CONCLUSIONS 94 I APPENDIX A. TABULATION OF FREQUENCY AND PERCENTAGE DISTRIBUTIONS BY CHARACTERISTICS 95 APPENDIX B. WORKING PAPER FORMS 255 Figure l I Figure 2 I Figure 3 I Figure 4 Figure 5 I Figure 6 Figure 7 I Figure 8 I Figure 9 I Figure 10 I Figure 11 Figure 12 I Figure 13 I I LIST OF ILLUSTRATIONS Frequency of Sightings by Year for Object, Unit, and All Sightings 17 Distribution of Evaluations of Object, Unit, and All Sightings for All Years 18 Distribution of Object Sightings by Evaluation for All Years With Comparisons of Each Year for Each Evaluation Group • 19 Distribution of Object Sightings by Evaluation for All Years and Each Year 20 Distribution of Object Sightings by Evaluation Within Months for All Years 21 Distribution of Object Sightings by Certain and Doubtful Evaluations for All Years and Each Year • 22 Frequency of Object Sightings and Unknown Object Evaluations by Months, 1947-1952. 23 Distribution of Object Sightings by Sighting Reliability Groups With Evaluation Distributions for Each Group . 24 Distribution of Object Sightings Among the Four Sighting Reliability Groups for All Years and Each Year . 25 Distribution of All Sightings by Sighting Reliability Groups, Segregated by Military and Civilian Observers, With Evaluation Distribution for Each Segregation 26 Distribution of Object Sightings by Reported Colors of Object(s) With Evaluation Distribution for Each Color Group • 27 Distribution of Object Sightings by Number of Objects Seen per Sighting With Evaluation Distribution for Each Group 28 Distribution of Object Sightings by Duration of Sighting With Evaluation Distribution for Each Duration Group . 29 iii I I I LIST OF ILLUSTRATIONS (Continued) Figure 14 Distribution of Object Sightings by Months Among the Eight Duration Groups for All Years Figure 15 Distribution of Object Sightings by Shape of Object(s) Reported With Evaluation Distribution for Each Shape Group , Figure 16 Distribution of Object Sightings by Reported Speed of Object(s) With Evaluation Distribution for Each Speed Group . Figure 17 Distribution of All Sightings by Observer Location for All Years and Each Year Figure 18 Comparison of Known and Unknown Object Sightings by Color, 1947-1952 Figure 19 Comparison of Known and Unknown Object Sightings by Number of Objects per Sighting, 1947-1952 Figure 20 Comparison of Known and Unknown Object Sightings by Speed, 1947-1952 Figure 21 Comparison of Known and Unknown Object Sightings by Duration, 1947-1952 Figure 22 Comparison of Known and Unknown Object Sightings by Shape, 1947-1952 Figure 23 Comparison of Known and Unknown Object Sightings by Light Brightness, 1947-1952 Figure 24 Comparison of Monthly Distribution of Object Sightings Evaluated as Astronomical Versus Total Object Sightings Less Astronomical. Figure 25 Comparison of Monthly Distribution of Object Sightings Evaluated as Aircraft Versus Total Object Sightings Less Aircraft Figure 26 Comparison of Monthly Distribution of Object Sightings Evaluated as Balloon Versus Total Object Sightings Less Balloon, Figure 27 Comparison of Monthly Distribution of Object Sightings Evaluated as Insufficient Information Versus Total Object Sightings Less Insufficient Information Figure 28 Comparison of Monthly Distribution of Object Sightings Evaluated as Other Versus Total Object Sightings Less Other Figure 29 Comparison of Monthly Distribution of Object Sightings Evaluated as Unknown Versus Total Object Sightings Less Unknown Figure 30 Characteristics Profiles of Object Sightings by Total Sample, Known Evaluations, and Individual Known Evaluations, With Unknown Evaluations Superimposed , Figure 31 Frequency of Object, Unit, and All Sightings Within the U. S., 1947-1952, by Subdivisions of One Degree of Latitude and Longitude Figure 32 Distribution of Object Sightings by Evaluation for the Twelve Regional Areas of the U. S., With the Strategic Areas Located Figure 33 Comparison of Evaluation of Object Sightings in the Strategic Areas of the Central East Region Figure 34 Comparison of Evaluation of Object Sightings in the Strategic Areas of the Central Midwest Region Figure 35 Comparison of Evaluation of Object Sightings in the Strategic Areas of the Central Farwest Region Figure 36 Comparison of Evaluation of Object Sightings in the Strategic Areas of the South Midwest Region . Figure 37 Comparison of Evaluation of Object Sightings in the Strategic Areas of the South West Region iv I Page 30 I 31 I 32 33 I 34 35 I 36 I 37 38 I 39 40 I 41 42 I 43 I 44 I 45 ,- 46 47 I 48 I 49 50 I 51 I 52 I 53 I I I I I I LIST OF ILLUSTRATIONS (Continued) Figure 38 Comparison of Evaluation of Object Sightings in the Strategic Areas of the I South Farwest Region . 54 I Figure 39 Diagram of a Celestial Sphere. 56 Figure 40 Frequency of Object Sightings by Angle of Elevation of the Sun, Intervals of 10 Degrees of Angle. 57 I Figure 41 Frequency of Object Sightings by Local Sun Time, Intervals of One Hour 59 Table Object Sightings 60 I Table II Chi Square Test of Knowns Versus Unknowns on the Basis of Color 62 Table III Chi Square Test of Knowns Versus Unknowns on the Basis of Number 63 I Table IV Chi Square Test of Knowns Versus Unknowns on the Basis of Shape 64 Table V Chi Square Test of Knowns Versus Unknowns on the Basis of Duration of Observation 65 I Table VI Chi Square Test of Knowns Versus Unknowns on the Basis of Speed ti6 Table VII Chi Square Test of Knowns Versus Unknowns on the Basis of Light Brightness 67 I Table VIII Chi Square Test of Revised Knowns Versus Unknowns on the Basis of Color 70 Table IX Chi Square Test of Revised Knowns Versus Unknowns on the Basis of Number. 71 I Table X Chi Square Test of Revised Knowns Versus Unknowns on the Basis of Shape 72 Table XI Chi Square Test of Revised Knowns Versus Unknowns on the Basis of Duration of Ob~ervation 73 I Table XII Chi Square Test of Revised Knowns Versus Unknowns on the Basis of Speed 74 Table XIII Chi Square Test of Revised Knowns Versus Unknowns on the Basis of Light Brightness. 75 I I I I I I I I I v and vi I I I SUMMARY I I Reports of unidentified aerial objects (popularly termed "flying saucers" or "flying discs") have been received by the U.S. Air Force since mid-1947 from' many and diverse sources. Although there was no evidence that the ·unexplained reports of unidentified objects constituted I a threat to the security of the U.S., the Air Force determined that all rep,orts of unidentified aerial objects should be fovestigated and evaluated to d,etermine if "flying saucers". represented technological developments not known to this country. I In order to discover any pertinent trend or pattern inherent in the data, and to evaluate or explain any trend or pattern found, appropriate I methods of reducing these data from reports of unidentified aerial objects to a form amenable to scientific' appraisal were employed. In general, the I original data upon which this study was based consisted of impressions and interpretations of apparently unexplainable events, and seldom contained reliable measurements of physical attributes. This subjectivity of the data I presented a 'major limitation to the drawing of significant conclusions, but did not invalidate the application of scientific methods bf study. I The reports received by the U.S. Air Force on unidentified aerial objects were reduced to IBM punched-card abstracts of the data by means of logically developed forms and standardized evaluation procedures. Evaluation of sighting reports, a crucial step in the preparation of the data I for statistical treatment, consisted of an appraisal of the reports and· the subsequent categorization of the object or objects described in each report. A detailed description of this phase of the study stresses the careful I attempt to maintain complete objectivity and consistency. I Analysis of the refined and evaluated data derived from the original reports of sightings consisted of (I) a systematic attempt to ferret out any distinguishing characteristics _inherent in the data of any of their segments, I (2) a concentrated study of any trend or pattern found, and (3) an attempt to determine the probability that any of the UNKNOWNS represent observa.:. tions of technological' developments not known to this country. I The first step in the 'analysis of the data revealed the existence of I certain apparent similarities between cases of objects definitely identified .i.nd those not identified. Statistical methods of testing when applied indicated a low probability that these apparent similarities were significant. An attempt to determine the probability that any of the UNKNOWNS represeri'ted observations of technological developments not known to this country necessi­ I tated a thorough re-examination and re-evaluation of the cases of objects not originally identified; this led to the conclusion that this probability was very small. I I The special study which resulted in this report (Analysis of Reports of Unidentified Aerial Objects, 5 May 1955) started in 1953. To provide the study group with a complete set of files, the informatio~ cut-off date was established as of the end of 1952. It will accordingly be noted that the statistics contained in all charts and tables in this report are terminated I vii !I I I I with the year 1952. In these charts, 3201 cases have been used. I As the study progressed, a constant program was maintained for the purpose of making comparisons between the current cases received after I January 1953, and those being used for the report. This was done I in order that any change or significant trend which might arise from current developments could be incorporated in the summary of this report. I The 1953 and 1954 cases show a general and expected trend of increasing percentages in the finally identified categories. They also.show decreasing percentages in categories where there was insufficient informa­ I tion and those where the phenomena could not be explained. This trend had been:. anticipated in the light of improved reporting and investigating pro­ ceduJ;"es. I Official reports on hand at the end of 1954 totaled 4834. Of these, 425 were produced in 1953 and 429 in 195'!. These 1953 and 1954 indi­ I vidual reports (a total of 854), were evaluated on the same basis as were those received before the end of 1952. The results are as follows: I Balloons - 16% Aircraft - 20% I Astronomical - 25% Other - 13% I Insufficient Info 1 7% 1. Unknown - 9% I As the study of the current cases progressed, it became increasingly obvious that if reporting and investigating procedures could be further improved, the percentages of those cases which contained insufficient information and I those remaining unexplained would be greatly reduced. The key to a higher percentage of solutions appeared to be in rapid "on the spot" investigations by trained personnel. On the basis of this, a revised program was estab- I lished by AF Reg. 200-2 Subject: "Unidentified Flying Objects Reporting" (Short Title: UFOB) dated 12 August 1954. I This new program, which had begun to show marked results before January 1955, provided primarily that the 4602d Air Intelligence Service' Squadron (Air Defense Command) would carry out all field investigations. I This squadron has sufficient units and is so deployed as to be able to arrive "on the spot" within a very short time after a report is received. After treatment by the 4602d AISS, all information is supplied to the Air Technical I Intelligence Center for final evaluation. This cooperative program has re­ sulted, since 1 January 1955, in reducing the insufficient information cases to 7o/o and the unknown cases to 3%, of the totals. I The period l January 1955 to 5 May 1955 accounted for 131 unidentified aerial object reports received. Evaluation percentages of these are as follows: I I viii I I I - I Balloons I Aircraft I ..... Astronomical Other I Insufficient Info - 7% I ,, Unknown - 3% I All available data were included in this study which was prepared by a panel of scientists both in and out of the Air Force. On the basis of this study it is believed that all the unidentified aerial objects could have been explained if more complete observational data had been available. Insofar fi as the reported aerial objects which still remain unexplained are concerned, there exists little information other than the impressions and interpretations I I of their observers. As these impressions and interpretations have been replaced by the use of improved methods of investigation and reporting, and ,by scientific analysis, the number of unexplained cases has decreased rapidly I towards the vanishing point. Therefore, on the b_asis of this evaluation of the information, it is r· I considered to be highly improbable that reports of unidentified aerial objects examined in this study represent observations of technological developments outside of the range of present-day scientific knowledge. It is emphasized that there has been a complete lack of any valid evidence of physical 'matter I in any case of a [1 I I ' I I ' f I I I I I - 26% - 21% - 23% 20% reported unidentified aerial object. ix I I INTRODUCTION I In June, 1947, Kenneth Arnold, a Boise, Idaho, businessman and I private pilot, publicly reported the now-famous sighting of a chainlike I formation of disc-shaped objects near Mount Rainier, Washington. Result­ ing newspaper publicity of this incident caught the public interest, and, shortly thereafter, a rash of reports of unidentified aerial objects spawned the term "flying saucers". During the years since 1947, many reports of I unidentified aerial objects have been received by the Air Force from many and diverse sources. I The unfortunate term "flying saucer", or "flying disc", because of I its widespread and indiscriminate use, requires definition. Many defini­ tions have been offered, one of the best being that originated by Dr. J. Allen Hynek, Director of the Emerson McMillin Observatory of The Ohio I State University, who has taken a scientific interest in_the problem of unidentified aerial objects since 1949. Dr. Hynek' s definition of the term ·1 is "any aerial phenomenon or sighting that remains unexplained to the viewer at least long enough for him to write a report about it11 ( 1). Dr. Hynek, I elaborating on his definition, says, "Each flying saucer, so defined, has associated with it a probable lifetime. It wanders in the field of public in­ spection like an electron in a field of ions, until I captured' by an explana­ tion which puts an end to its existence as a I flying saucer' 11 ( 1). I This definition would be applicable to any and all of the sightings I which remained unidentified throughout this study. However, the term "flying saucers" shall be used hereafter in this report to mean a novel, airborne phenomenon, a manifestation that is not a part of or readily ex­ I plainable by the fund of scientific knowledge known to be possessed by the Free World. This would include such items as natural phenomena that are not yet completely understood, psychological phenomena, or intruder air­ craft of a type that may be possessed by some source in large enough I numbers so that more than one independent mission may have been flown I and reported. Thus, these phenomena are of the type which should have been observed and reported more than once. I Since 1947, public interest in the subject of unidentified aerial objects fluctuated more or less within reasonable limits until the summer of 1952, I when the frequency of reports of sightings reached a peak, possibly stimu­ lated by several articles on the subject in leading popular magazines. I Early in 1952, the Air Force's cumulative study and analysis of reported sightings indicated that the majority of reports could be accounted for as misinterpretations of known objects ( such as meteors, balloons, or aircraft), a few as the result of mild hysteria, and a very few as the result I of unfamiliar meteorological phenomena and light aberrations. However, I (1) Hynek, J. A.• "Unusual Aerial Phenomena"• Journal of the Optical Society of America. 43 (4). pp 311-314, April, 1953. J 1 I I a significant number of fairly complete reports by reliable observers re­ mained unexplained. Although no evidence existed that unexplained reports of sightings constituted_ a 1>hysical threat to the security of the U. S., in I March, 1952, the Air Force decided that all reports of unidentified aerial objects should be investigated and evaluated to determine if "flying saucers" represented technological developments not known to this country. I Originally, the problem involved the preparation and analysis of about I 1,300 reports accumulated by the Air Force between 1947 and the end of March, 1952. During the course of the work, the number of reports sub­ mitted for analysis and evaluation more than tripled, the result of the un­ I precedented increase in observations during 1952. Accordingly, this study is based on a number of reports considered to be large enough for a pre­ liminary statistical analysis, approximately 4, 000 reports. I This study was undertaken primarily to categorize the available I reports of sightings and to determine the probability that any of the reports of unidentified aerial objects represented observations of "flying saucers". With full cognizance of the quality of the data available for study, yet with I an awareness of the proportions this subject has assumed at times in the public mind, this work was undertaken with all the seriousness accorded I to a straightforward scientific investigation. In order to establish the probability that any of the reports of unidentified aerial objects represented observations of "flying saucers", it was necessary to make an attempt to I answer the question "What is a 'flying saucer'?". However, it must be emphasized that this was only incidental to the primary purpose of the study, the determination of the probability that any of the reports of un­ I identified aerial objects represented observations of "flying saucers", as defined on Page 1. I The basic technique for this study consisted of reducing the available data to a form suitable for mechanical manipulation, a prerequisite for the I application of preliminary statistical methods .. One of International Business Machine Corporation's systems was chosen as the best available mechanical equipment. I The reduction of data contained in sighting reports into a form suit­ I able for transfer to IBM punched cards was extremely difficult and time consuming. I For this study a panel of consultants was formed, consisting of both experts within and outside A TIC. During the course of the work, guidance I and advice were received from the panel. The professional experience available from the panel covered major scientific fields and numerous specialized fields. I All records and working papers of this study have been carefully preserved in an orderly fashion suitable for ready reference. These I I 2 l I I records include condensations of all individual sighting reports, and the IBM cards used in various phases of the study. I I ORIGIN AND NATURE OF DA TA I I Reports of sightings were received by the U. S. Air Force from a representative cross section of the population of the U. S., and varied widely in completeness and quality. Included were reports from reputable scientists, housewives, farmers, students, and technically trained mem­ I bers of the Armed Forces. Reports varied in length from a few sentences stating that a "flying saucer" had been sighted, to those containing thou­ sands of words, including description, speculation, and advice on how to I handle the "problem of the I flying saucers 1 ". Some reports were of high I quality, conservative, and as complete as the observer could make them; a few originated from people confined to mental institutions. A critical examination of the reports revealed, however, that a high percentage of them was submitted by serious people, mystified by what they had seen and I motivated by patriotic responsibility. Three principal sources of reports were noted in the preliminary I review of the data. The bulk of the data arrived at A TIC through regular military channels, from June, 1947, until the middle of 1952. I A second type of data consisted of letters reporting sightings sent by civilian observers directly to A TIC. Most of these direct communications I were dated subsequent to April 30, 1952, and are believed to be the result I of a suggestion by a popular magazine that future reports be directed to the Air Technical Intelligence Center. As could be expected, a large number of letters was received following this publicity. I A third type of data was that contained in questionnaire forms com­ pleted by the observer himself. A questionnaire form, developed during the course of this study, was mailed by A TIC to a selected group of writers I of direct letters with the request that the form be completed and returned. Approximately 1,000 responses were received by ATIC. I In general, the data were subjective, consisting of qualified estimates I of physical characteristics rather than of precise measurements. Further­ more, most of the reports were not reduced to written form immediately. The time between sighting and report varied from one day to several years. Both of these factors introduced an element of doubt concerning the validity I o,f the original data, and increased its subjectivity. This was intensified by I the recognized inability of the average individual to estimate speeds, dis­ tances, and sizes of objects in the air with any degree of accuracy. In spite of these limitations, methods of statistical analysis of such reports m sufficiently large groups are valid. The danger lies in the possibility of I 3 I I I forgetting the subjectivity of the data at the time that conclusions are drawn from the analysis. It must be emphasized, again and again, that any conclusions contained in this report are based NOT on facts, but on what I many observers thought and estimated the true facts to be. Altogether, the data for this study consisted of approximately 4, 000 I reports of sightings of unidentified aerial objects. The majority were :re­ ceived through military channels or in the form of observer-completed I questionnaires; a few were accepted in the form of direct letters from un­ -questionably reliable sources. Sightings made between June, 1947, and December, 1952, were considered for this study. Sightings alleged to have I occurred prior to 1947 were not considered, since they were not reported to official sources until after public interest in "flying saucers" had been stimulated by the popular press. I I REDUCTION OF DA TA TO MECHANIZED COMPUTATION FORM I As received by the Air Technical Intelligence Center, the sighting reports were not in a form suitable for even a quasi-scientific study. A I preliminary review of the data indicated the need for standardized interro­ gation procedures and supplemental forms for the reduction of currently I held and subsequently acquired data to a form amenable to scientific appraisal. I The plan for reduction of the data to usable form consisted of a pro­ gram of development comprising four major steps: ( 1) a systematic listing I of the factors necessary to evaluate the observer and his report, and to identify the unknown object observed; (2) a standard scheme for the trans­ fer of data to a mechanized computation system; (3) an orderly means of I relating the original data to_ all subsequent forms; and (4} a consistent pro­ cedure for the identification of the phenomenon described by the original data. I I Questionnaire I I The first reports received by A TIC varied widely in completeness and quality. Air Force Letter 200-5(2} and Air Force Form 112( 1) were I attempts to fix responsibility for and improve the quality of the reports of sightings. To coordinate past efforts and to provide standardization for the I (1) A modified Air Force Form 112 lists pertinent questions to be answered in regard to an unidentified-object sighting. (2) Air Force Letter 200-5 places responsibility with the Air Force for the investigation, reporting, and I analysis of unidentified aerial objects. This letter is dated 29 April 1952. I 4 I I I I future, it was imperative to develop a questionnaire form listing the factors necessary for evaluation of the observer and his report, and identification of the unknown objects. In addition, it was decided that such a questionnaire should be designed to serve as an interrogator's guide, and as a form for I the observer himself to complete when personal interrogation was not possi­ ble or practicable. I Ideally, a questionnaire for the purposes required should contain I questions pertaining to all technical details considered to be essential for the statistical approach, and should serve to obtain a maximum of informa­ tion from the average individual who had made a sighting in the past or would be likely to be reporting sightings in the future. Besides these ~is­ I crete facts, an integrated written description of a sighting would be re­ quired, thus enabling the reported facts of the sighting to be corroborated. I Also, a narrative description might allow subtle questions to be answered concerning the observer's ability, such as indirect questions that would I reveal his reasoning ability, suggestibility, and general mental attitude. As a whole, then, the information contained in a questionnaire should make possible the classification and evaluation of the sighting, the rating of the observer, the probability of accuracy of reported facts, and the identifica­ I tion of what was reported by the observer as unidentified. I During the course of this project, three questionnaire forms were I developed, each intended to be an improved revision of the one preceding. The improvements were suggested and confirmed by members of the panel of consultants connected with this project. I The original form was evolved by the panel of consultants as their first work on this project. It was intended to allow the start of the reduc­ tion of reports to discrete data, and was immediately subjected to exten­ I sive review and revision by the panel. The revised ( second) form was subjected to a trial test before adoption. A TIC sent a copy to observers reporting sightings, with the request that the form be completed and re­ I turned. Of the first 300 questionnaires returned during July and August, I 1952, 168 were analyzed by a consulting psychologist. On the basis of this analysis, plus the experience gained in working with past reports, the final form of the questionnaire - the U. S. Air Force Technical Information I Sheet - was evolved. Copies of the three forms of the questionnaire, in the order of their development, are shown as Exhibits BI, B2, and B3 in Appendix B. I In order to implement the transcription of data from past sighting I reports, each succeeding form was put to use as soon as it was developed and approved. Accordingly, experience was obtained with each form in relation to past data, an important factor in the improvement of the quality and completeness of the later reports included in this study. I I 5 I I I Coding System and Work Sheet I The reduction of non-numerical data to numerical form is mandatory in the machine handling of data. Thus, the selection of the IBM punched­ card system for analysis of data forced the adoption of a master coding I plan. Since it was impracticable to transfer detailed data of a~ exact nature from the questionnaire to the IBM card, an intermediate transfer I form, coordinated with the master code, was necessary. The master coding plan was evolved during the early stages of the I preli~inary analysis of data, and was reviewed by the panel of consultants before use. It was recognized that this system of coding would be the heart of the analysis, that is, the completeness of the facility for trans­ I lation of data could make or break the study. Accordingly, every conceiv­ able factor that might influence the identification of unidentified aerial I objects was included, together with a wide range of variations within each factor. The original coding system (with minor corrections) was used throughout the translation of the original data with marked success. A copy I of this system, called CODES, is enclosed as Exhibit IH, Appendix B. I To facilitate the preparation of the punched-card abstract, an inter­ mediate form called the WORK SHEET (later, the CARD BIBLE) was developed. Referenced to both the data from the questionnaire and the sys­ I tem of report identification, the WORK SHEET permitted an orderly transcription of data simultaneously by several people. In conjunction with the CODES, the WORK SHEET was used during the reduction of the I original data to code form necessary for transfer to punched cards. A sample is included as Exhibit BS, Appendix B. I After the analysis was under way, it became apparent that the me­ chanics of machine processing could be improved by incorporating in the I IBM card system group classifications of certain factors requiring more than one column for discrete expression. In addition, the inclusion of certain data relating to the evaluation and bearing of the sun with respect I to the observer was considered necessary. Finally, a critical examination of certain segments of the data indicated the need for the definition of a I new factor relating to the maneuvers of the object or objects sighted. Prior to the start of the analytical study, it had been assumed that a com­ bination of stated factors would, by inference, define the maneuver pattern. I All these additions have been incorporated in a revised set of CODES I and CARD BIBLE that are illustrated as Exhibits B6 and B7, Appendix B. However, at the time that the maneuver factor was determined to be criti­ cal, it was physically impracticable to make the required definitions and I re-evaluate the original data. Therefore, no code for maneuverability has been included in the CODES, CARD BIBLE, or IBM cards. I I 6 I I I Identification of Working Papers I I The actual reduction of data to IBM punched-card form presented a problem of mass transfer of figures by several workers. Recognizing that an orderly system of relating the original data to the questionnaire, the WORK SHEET, and the IBM card was imperative, a scheme of SERIAL I NUMBERS was developed to answer this need. I The first data consisted of a series of letter-file folders identified by the year and location of the sighting or sightings they contained. The num­ I ber of reports of sightings in a single folder varied from 1 to over 20. Under these conditions, there was a great possibility for incorrect tran­ scription of data, duplication of transcription, or misplacement of inter­ mediate forms. Further, it was considered desirable to relate all sightings I of the same object or objects to one another. The concept of a four-digit I serial number (major), followed by a two-digit subserial number (minor), was adequate to fulfill these requirements. To expedite handling of the data, temporary serial numbers were I assigned until each report had been evaluated and the phenomenon had been I placed i11:, a category of identification. The use of temporary serial num­ bers permitted the consolidation of duplicate reports from apparently diverse sources, such as a teletype message and an Air Force Form 112. I However, this consolidation was made ONLY when it could be proved con­ clusively that the sources of the two documents were one and the same. Factors of the observer's location, date and time of observation, descrip­ tion of the phenomenon, and finally, the name of the observer were con­ I sidered. In this manner, the assignment of major serial and minor sub­ I serial numbers in continuous series was made only to the reports accepted for the statistical study. It is believed that the reports accepted represent unique and unduplicated instances of sightings. I In the establishment of the serial-number system, it was necessary to define certain terms,' so that a standard interpretation could be achieved. The terms and corresponding definitions were: I OBSERVER - I SIGHTING I I I I I Any witness reporting to a proper authority that he had seen unidentified aerial objects. The report or group of reports of the same observed phenomenon that remained unidenti­ fied to the observer or observers, at least until reported. 7 I I SINGLE OBSERVATION - A SIGHTING consisting of a single report from ( 1) one OBSERVER with no knowledge of additional OBSERVERS of the same phenom­ I enon, or (2) a group of witnesses of the same phenomenon, each cognizant of the others. The witness who made the report is called a SINGLE I OBSERVER. I MULTIPLE OBSERVATION - A SIGHTING consisting of several reports from OBSERVERS of the same phenomenon who were cognizant of each other. I The witnesses who made reports are called MULTIPLE OBSERVERS. I ALL SIGHTINGS - ( 1) The group of reports consisting of one report for each OBSERVER, including both I SINGLE and MULTIPLE OBSERVERS. (2) The questionnaire, work sheet, and IBM card representing the report from each OBSERVER - I in other words, the representation of each report accepted for the statistical study. I UNIT SIGHTINGS - ( 1) The group of reports consisting of one report for each SIGHTING, including all the I reports of SINGLE OBSERVATIONS and the one most representative report from each MULTIPLE OBSERVATION. (2) The questionnaire, work I sheet, and IBM card representing the report for each SIGHTING accepted for the statistical study. I A major serial number (four digits) was assigned to each sighting, segregating the year of occurrence by selection of limits for each year, as I follows: I 0001 to 0500 reserved for 1947 0501 to 1000 reserved for 1948 1001 to 1500 reserved for 1949 I 150lto2000 reserved for 1950 2001 to 2500 reserved for 1951 2501 to4900 reserved for 1952 I While this scheme would serve to identify any individual sighting, identifi­ I cation of each report and its subsequent forms was necessary. The minor subserial numbers (two digits) fulfiiled this requirement. For all SINGLE OBSERVATIONS, a major serial number followed by two (2) zeros, for I example, 2759. 00, was sufficient identification. For MULTIPLE OBSER­ VATIONS, the major serial number followed by a series of two-digit num­ bers ranging from 00 to 99 was used to identify the individual reports. In I general, the most complete report from the most reliable observer of that I 8 I I I MULTIPLE OBSERVATION was identified with the . 00 subserial number. As an example, a MULTIPLE OBSERVATION consisting of six sighting I reports would have the following serial numbers: I 1132. 00 representing the best report and observer 1132.01 representing an additional observer 1132. 02 representing an additional observer I 1132. 03 representing an additional observer I 1132. 04 representing an additional observer 1132. 05 representing an additional observer I During the course of the transcription of the data to machine card form, it became obvious that certain reports could have been independent observations of the same phenomenon. So, if the presentation of an analysis based on one report for each sighting was valid (the concept of I UNIT SIGHTINGS}, a presentation of an analysis based on one report for each phenomenon should be valid also. Further, the examination of data I relating to the actual number of phenomena was considered to be the proper basis for assessing the probability of technological developments outside the range of present-day scientific knowledge. Therefore, a designation of I OBJECT SIGHTINGS was established, with the following definition: OBJECT SIGHTING - ( 1) The group of reports consisting of I one report for each phenomenon. (2) The I questionnaire, work sheet, and IBM card representing a report for each phenomenon accepted for the statistical study. I In brief review, ALL SIGHTINGS refer to all reports, UNIT SIGHTINGS refer to actual sightings, and OBJECT SIGHTINGS refer to the assumed I number of phenomena. I It must be recognized that the process of identifying OBJECT SIGHTINGS was deductive, while that for UNIT SIGHTINGS was definitive. A conservative approach was adopted in the determination of OBJECT SIGHTINGS, using the factors of date and time of observations, location I of observers, duration of observations, and range, bearing, track direc­ I tion, and identification of the phenomena. Any error of selection of OBJECT SIGHTINGS will tend to be in the direction of reducing the actual number of phenomena observed ( several instances of UNIT SIGHTINGS that might be one OBJECT SIGHTING were noted, but the evidence was not conclusive I enough to justify consolidation of the reports). I Following the determination of OBJECT SIGHTINGS, a series of serial numbers, called the INCIDENT SERIAL NUMBERS, was established I to facilitate any future study of a specific object sighting. Each reported sighting that relates to an OBJECT SIGHTING received the same incident serial number, a four-digit code paralleling the major serial number series. I 9 I I I For machine manipulation, it was desirable to be able to select the sample of cards (all reports, all sightings, or all phenomena) to be in­ cluded in a particular study. The concept of a SIGHTING IDENTIFICATION I NUMBER was evolved to fill this desire. Using one column of the IBM card, and the correlated working papers,· the code for this function was developed. Multiple punching eliminated the need to use several columns I for discrete expression of the variations. Selection of the proper number in this column thus permitted selection of the desired sample of cards. ·1 Evaluation of Individual Reports I I Evaluation of sighting reports was recognized as a crucial step in the preparation of data for statistical treatment; inconsistent evaluations would have invalidated any conclusions to be derived from this study. A method I of evaluation was, therefore, determined simultaneously with the develop­ ment of the questionnaire, the coding system, and the work sheet. It is emphasized that all phases of evaluation, even including the tedious prep­ I aration of the original data for statistical treatment, were entrusted only to selected, specially qualified scientists and engineers. I Evaluation consisted of a standardized procedure to be followed for: ( 1) the deduction of discrete facts from data which depended on human im­ I pressions rather than scientific measurements, (2) the rating of the ob­ server and his report as determined from available information, and ( 3) the determination of the probable identification of the phenomenon observed. I Categories of identification, established upon the basis of previous experi­ ence, were as follows: I Balloon Astronomical I Aircraft Light phenomenon Birds I Clouds, dust, etc. Insufficient information I Psychological manifestations Unknown Other I The first step in evaluation, the deduction of discrete facts from I subjective data, required certain calculations based on the information available in the sighting report. An example was the finding of the approxi­ mate angular velocity and acceleration of the object or objects sighted. I Care was taken during this phase of the work to insure against the deduc­ tion of discrete facts not warranted by the original data. Thus, even though there was a complete lack of any valid evidence consisting of I I 10 I I I physical matter in any case of a reported unidentified aerial object, this I was not assumed to be prima facie evidence that "flying saucers" did not exist. I In those cases in which an attempt to reduce the information to a 1· factual level failed completely, the report was eliminated from further con­ sideration, and thus not included in the statistical analysis. About 800 I reports of sightings were eliminated or rejected in this manner. Most of these reports were rejected because they were extremely nebulous; the rest were rejected because they contained highly conflicting statements. The second step in evaluation, the rating of the observer and his I report, logically followed the first step, the reduction of the data to usable form. Ratings were assigned on the basis of the following factors of in­ I formation, considered in relation to one another: I ( 1) The experience of the observer, deduced from his occupation, age, and training; ( 2) The consistency among the separate portions of the I description of the sighting; I (3) The general quality and completeness of the report; (4) Consideration of the observer's fact-reporting ability I and attitude, as disclosed by his manner of describing the sighting. I In cases in which insufficient information was available to make a judgment of the observer or report, none was made, but the report was accepted for I the statistical study. The third step in the process of evaluation, the attempted identifica­ I tion of the object or objects sighted, was done twice, first by the individual I who made the transcription of the data (the preliminary identification), and later (the final identification} by a conference of four persons, two repre­ sentatives from ATIC and two from the panel of consultants. Although representatives of A TIC participated in making the final identifications, it I must be emphasized that any previous identification of a sighting made by A TIC was not introduced or referred to in any way. I In the coding system, the choices provided for final identifications were based on A TIC' s previous experience in analysis of the data. They I had found that the majority of sightings could be classified as misinterpre­ tations of common objects or natural phenomena. Accordingly, categories I for objects most frequently present in the air were provided. Balloons, aircraft, astronomical bodies ( such as meteors), birds, and clouds or dust I were recognized as major categories. The less frequent, but common objects, such as kites, fireworks, flares, rockets, contrails, and 11 I I I meteorological phenomena like small tornadoes, were collected into a category called OTHER. A separate category for the uncommon natural phenomena associated with light reflections or refractions, such as mirages, I sun dogs, inversion-layer images, and distortions caused by airborne ic.e, was established with the title of LIGHT PHENOMENON. Categories for INSUFFICIENT INFORMATION, PSYCHOLOGICAL MANIFESTATIONS, and I UNKNOWN were provided for the sightings that could not be fitted into the preceding identifications. An explanation of their use follows: I INSUFFICIENT INFORMATION - This identification category was assigned to a report when, upon final con­ I sideration, there was some essential item of information missing, or there was enough doubt abou._t what data were available to disallow I •identification as a common object or some natural phenomenon. It is emphasized that this I category of identification was not used as a convenient way to dispose of what might be called "poor unknowns", but as a category for I reports that, perhaps, could have been one of several known objects or natural phenomena. No reports identified as INSUFFICIENT INFORMA­ I TION contain authenticated facts or impressions concerning the sighting that would prevent its I being identified as a known object or phenomenon; PSYCHOLOGICAL MANIFESTATIONS - This identification I category was assigned to a report when, although it was well established that the ob­ I server had seen something, it was also obvious that the description of the sighting had been overdrawn. Religious fanaticism, a I desire for publicity, or an over-active imagi­ nation were the most common mental aber­ rations causing this type of report; I UNKNOWN - This designation in the identification code was I assigned to those reports of sightings wherein the description of the object and its maneuvers could not be fitted to the pattern of any known I object or phenomenon. I For the purposes of this study, two groups of identifications were recognized, the KNOWNS (including all identification categories except the UNKNOWNS) and the UNKNOWNS. I All possible identifications provided in the code system, except INSUFFICIENT INFORMATION and UNKNOWN, could be assigned accord­ I ing to two degrees of certainty, designated "Certain" and "Doubtful". I I 12 I I I A "Certain" identification indicated a minimum amount of doubt regarding the validity of the evaluation. By "rule-of-thumb" reasoning, the proba­ bility of the identification being correct was better than 95 per cent. A "Doubtful'' identification indicated that the choice was less positive, but I that there was a better than even chance of being correct. I It is emphasized again that, as was tr.ue for other phases of evalua­ tion, preliminary and final identification was entrusted only to scientists and engineers who, in addition to their broad scientific background, had I received instruction, where necessary, in specialized subjects. The panel of consultants provided background information for this instruction. Many of the cases representing unusual features or maneuvers were submitted to I and discussed with various members of the panel of consultants prior to the final identification. I Consistency in the application of the knowledge necessary for making identifications was maintained by frequent collaboration among the person­ I nel involved, and systematic spot checks of the work. In addition to the general fund of knowledge required to identify satisfactorily a reported unidentified aerial object, an attempt was made to correlate specific data I such as flight plans of aircraft, records of balloon releases, weather con­ ditions, and an astronomical almanac with the reported sighting. I The procedure followed in making final identifications deserves ex­ planation because of the importance assumed by the identification as a basis I for statistical treatment. As was mentioned, a conference of four qualified I persons, two from A TIC and two from the panel of consultants, decided upon the final identification for each sighting report. This work was done at A TIC, periodically, as reports became ready. I During an identification conference, each sighting report was first studied, from the original data, by one person. If that person arrived at a decision, it was checked against the preliminary identification; if the two I identifications were the same, the report was appropriately marked and I considered finished. If the two identifications did not agree, the report was considered later by everyone participating in the conference until a group decision could be made. I If an evaluator was unable to categorize the report as one of the common objects or as a natural phenomenon, and his opinion was that the sighting should be recorded as UNKNOWN, a group decision was also re­ I quired on that report before it was considered finished. A group decision I was necessary on all reports finally recorded as UNKNOWN, regardless of what the preliminary identification had been. In cases where a group I decision was not made within a reasonable time, the report was put aside and later submitted to certain members of the panel of consultants for their op1n1ons. If, after this, disagreement continued to exist, the report of the sighting was identified as UNKNOWN. I 13 I I I Upon completion of final identifications, all data were transferred to IBM cards, preparatory to analysis. I I I ANALYSIS OF THE DATA Broadly stated, the problem at this point consisted of the judicious application of scientific met~ods of categorizing and analyzing the sub­ jective data in reports of sightings of unidentified aerial objects. It was I recognized that an approach to this problem could best be made by a sys­ tematic sorting and tabulation program to give frequency and percentage I distributions of the important characteristics of sightings. A suggestion that an attempt be made to anticipate all questions that might be asked in the future about a sighting or a group of sightings, and to provide answers, I was rejected. The systematic approach also made it possible to develop a detailed reference manual of the attributes of the sightings included in this study. I Thus, at the beginning of the analysis, a detailed plan was developed I for sorting, counting, and tabulating the information from the punched-card abstracts of reports of sightings. It was believed at the time, and later substantiated, that the results of the program for sorting and tabulating I would serve as a, guide for the more sophisticated treatment involving statistical methods. I Also, it was anticipated that any patterns or trends that might be found could be subjected to concentrated study in the hope of discovering I significant information relating to the characteristics of "flying saucers". Further, it was believed that these trends could serve as certain of the criteria of validity for any concepts (models) developed in the attempt to I discover a class of "flying saucers". I The three parts of this study ( 1) the sorting and tabulation program, (2) the advanced study of the results of that program, and (3) the investiga­ tion of the possibility of conceiving a model of a "flying saucer" from I descriptions reported, are discussed in sections entitled "Frequency and Percentage Distributions by Characteristics", "Advanced Study of the Data", and "The I Flying Saucer' Model". I I Frequency and Percentage Distributions by Characteristics I The original conception of this study assumed the availability of sufficient data to describe adequately the physical appearance, maneuver characteristics, range, direction, and probable path of the object or I objects observed. However, familiarity with the data, acquired during the I 14 I I ~ I translation and transcription from reports to punched cards, indicated that there would be relatively few specific variables or factors that would yield meaningful correlation studies. Either the original data were too subjec­ \L, tive, or the incompleteness of the original reports-wo1J:_ld seriously reduce the sample of a specific variable. I I Preliminary tabulations of various sortings substantiated the im­ possibility of deriving statistical results from certain variables, such as movement of the observer during the sighting, sound, shape parameter, I size, angular velocity and acceleration, appearance and disappearance bearing, initial and final elevation, altitude, and orientation of the object. The statistically usable variables presented in this study include the date, I time, location, duration, reliability, and method of observation of the sighting, and the physical attributes of number, color, speed, shape, light I brightness, and identification of the objects sighted. The presentation of frequency and percentage distributions of any of I the variables must be interpreted in the light of the sample of incidents I represented. For example, the analysis of the reported colors of the objects sighted, based on ALL SIGHTINGS, could lead to misrepresenta­ tion of the distribution of the reported color of the objects, because of the I multiplicity of reports on some of the phenomena. On the other hand, the percentage distribution of the light brightness reported by each observer is more likely to be correct than a distribution based on one report for each phenomenon. To assure that the most nearly correct presentation I was made, and to avoid the possibility· of failure to uncover any pattern or I trend inherent in the data, the variables were studied on five different bases or samples. These samples, and their numerical relation to each other, were as follows: I ALL SIGHTINGS (all reports) 3,201 cards UNIT SIGHTINGS, all observers 2, 554 cards UNIT SIGHTINGS, single observer 2, 232 cards I UNIT SIGHTINGS, multiple observers 322 cards OBJECT SIGHTINGS 2, 199 cards I The preliminary tabulations indicated that the samples based on UNIT SIGHTINGS, single observer, and UNIT SIGHTINGS, multiple observers, I would not add materially to this study. Accordingly, although the fre­ I quency distributions were recorded and are available for study, they are not presented in this report. 'I The bases of ALL SIGHTINGS, UNIT SIGHTINGS (referring to all observers), and OBJECT SIGHTINGS are presented in Appendix A as Tables Al through A240. A critical study of these tabulations reveals that there is no apparent change in the distribution of any variable from one I basis to another, and that no marked patterns or trends exist in any sample. _I 15 I I I Graphical Presentation Graphical representation of the important information contained in I the tables is presented in Figures l through 38. These figures present the distributions of the important variables only by the total nu.mber of cases I in each identification category, since no significant differences were found between the distributions of "Certain'' and "Doubtful" identifications of objects with respect to the variables. A chronological study of these I figures will afford a broad picture of the tabulated information, without the necessity of a detailed study of the tables. I A critical examination of the figures will show that no trends, patterns, or correlations are to be found, with the exception of Figures 18 through 30. I The apparent similarity of the distributions shown by these mirror graphs, Figures 18 through 23, was tested by statistical methods which showed that there was a low probability- that the distributions of the KNOWNS, and I UNKNOWNS by these characteristics were the same. These tests and their interpretation are discussed in the following section. For purposes of this I study, the strategic areas, shown in Figures 32 through 38, and Tables A223 through A240, Appendix A, were designated on the basis of concen­ tration of reports of OBJECT SIGHTINGS in an area. No other interpre­ I tation of the tables or remaining charts was deemed necessary. I Advanced Study of the Data I It was recognized that the lack of any patterns or trends, as shown by the tabulations and graphs, provided an insecure basis for drawing definite I conclusions. Accordingly, shortly before the sorting and tabulation pro­ gram was concluded, a program of study of the data was developed to utilize statistical and other mathematical methods, which could lead to a I more concrete interpretation of the problem. I Position of the Sun Relative to the Observer I The first thing that was done was to calculate the angle of elevation of the sun above the horizon and its bearing from true north as seen by the I observer at the time of each sighting. With this information, it could then be determined whether there was a possibility that the reported object could have been illuminated by light from the sun. In addition, it could be I determined whether an object could be a mock sun ( sun dog) or whether there was a possibility of specular reflection from an aircraft at the posi­ tion of the object, which would give the appearance of a "flying disc". I A program of computation was set up and carried out to obtain the I angle of elevation and the bearing of the sun for each sighting. All informa­ tion needed for this calculation was available on the deck of IBM cards. I 16 I I I I I I I I I I I ,1952, 1501= 68.3% I I I I I I I Object sightings 2199 =100% I FIGURE I FREQUENCY OF SIGHTINGS BY YEAR FOR OBJECT, UNIT, AND ALL SIGHTINGS I A-7479 I I 17 I ~ All sightings 3201 = 100% Unit sightings 2554 =100% I­ I I I I I I I I I 474 =21.5% I I I I I I Object sightings All sightings Unit sightings 2199=100 % 3201=100% 2554= 100% I FIGURE 2 DISTRIBUTION OF EVALUATIONS OF OBJECT, UNIT, AND ALL SIGHTINGS FOR ALL YEARS I A-7480 I I I 18 I I I I I I I I I I I I I I I I 1952 Unknown 434=19.7°1. Astronomical 479 =21.8 % Aircraft I I FIGURE 3 DISTRIBUTION OF OBJECT SIGHTINGS BY EVALU­ ATION FOR ALL YEARS WITH COMPARISONS OF EACH YEAR FOR EACH EVALUATION GROUP I A•7411 I I I 19 11 ---------------------- 100 No. of object sightings 100 2199 79 143 186 169 121 1501 90 90 80 80 70 70 60 60 +­c ~ 50 50 ~ Q,J ro 0 a. 40 40 30 30 20 20 10 10 C\\.\.:::<::<>0)J L>,,),,),,),,\.'\'\1 l'\;\'\\:\,'\'\'1 I\'\»),),).);! N>,'\',,'\'\>)j I\Y\'\\.'\'\"l RL~ I 0 - •• 1947 1948 1949 1950 1951 1952 0 I FIGURE 4 DISTRIBUTION OF OBJECT SIGHTINGS BY EVALUATION FOR ALL YEARS AND EACH YEAR A-7 4 82 ---------------------- No. of object sightings 70 55 83 127 129 183 638 407 166 125 106 105 I F+--1 ;..I 11 I I 11 11 I I I 11 11 I I I I I I I I 11 11 I I I I I I I I 11 11 I I 11 I I I I 11 11 I I 11 I I I I II 11 I I 11 1100 90 "' Cl ·= +- 80 80 .c Cl cn 7 0 70 +­u cu :.0 60 60 0 .c"' c 50 50 0 ~ ~ 40 40 "' 0 LLJ- 0 30 30 +­c cu u 20 20 '­cu a. 10 10 0 I l>.':u>l I\»'\'I l>,V0J l\'\:0::1 ~ ~ l'\'001 J:00-:\'I L),'\).)I ~J l'\'\)-'::J l\»>.J I 0 Jon Feb Mor Apr Moy June July Aug Sept Oct Nov Dec FIGURE 5 DISTRIBUTION OF OBJECT SIGHTINGS BY EVALUATION WITHIN MONTHS FOR ALL YEARS A-7413 30 20 10 0 10 20 30 I I I I I I I I I I I I Unknown I All years I I 1947 I I 1948 I 1949 I 1950 I 195 I I 1952 I Other Astronomical I All years I 1947 I I I 1948 I I 1949 I I 1950 I 1951 I I I 195 2 I. Aircraft I All years I I 1947 I I 1948 I I 1949 I I 1950 I I 195 I I I 195 2 I Balloons All years I I 1947 I 1948 I I 1949 I I 1950 I I 195 I I I 195 2 I I lnsuf. info. I All years I 1947 I 1948 I 1949 I 1950 I 195 I I 195 2 All years I 1947 1948 I 1949 1950 195 I 1952 I 30 20 10 0 10 20 30 I Certain Per