Identifying unidentified flying objects (UFOs) is a difficult task due to the normally poor quality of the evidence provided by those who report sighting the unknown object. Observations and subsequent reporting are often made by those untrained in astronomy, atmospheric phenomena, aeronautics, physics, and perception. Nevertheless, most officially investigated UFO sightings, such as from the U.S. Air Force's Project Blue Book, have been identified as being due to honest misidentifications of natural phenomena, aircraft, or other prosaic explanations. In early U.S. Air Force attempts to explain UFO sightings, unexplained sightings routinely numbered over one in five reports. However, in early 1953, right after the CIA's Robertson Panel, percentages of unexplained sightings dropped precipitously, usually being only a few percent in any given year. When Project Blue Book closed down in 1970, only 6% of all cases were classified as being truly unidentified.

UFOs that can be explained are sometimes termed identified flying objects (IFOs).

UFO studies

The following are some major studies undertaken since 1948 that reported on identification of UFOs: Project Sign Project Blue Book Special Report No. 14 (referred to further below as BBSR) was a massive statistical study the Battelle Memorial Institute did for the USAF of 3,200 UFO cases between 1952 and 1954. Of these, 22% were classified as unidentified ("true UFOs"). Another 69% were deemed identified (IFOs). There was insufficient information to make a determination in the remaining 9%. The official French government UFO investigation (GEPAN/SEPRA), run within the French space agency CNES between 1977 and 2004, scientifically investigated about 6000 cases and found that 13% defied any rational explanation (UFOs), while 46% were deemed readily identifiable and 41% lacked sufficient information for classification. The USAF-sponsored Condon Committee study reported that all 117 cases studied were or could probably be explained. A 1971 review of the results by the American Institute of Aeronautics and Astronautics concluded that 30% of the 117 cases remained unexplained. Of about 5,000 cases submitted to and studied by the civilian UFO organization NICAP, 16% were judged unknowns. The Pentagon UFO Report, is a United States federally mandated assessment summarizing information regarding unidentified flying objects (UFOs), also known as unidentified aerial phenomenon (UAPs). On June 25, 2021, a nine-page preliminary assessment was issued. It states that the UAPTF (Unidentified Aerial Phenomena Task Force) focused on 144 observations of "unidentified aerial phenomena" by the U.S Armed Forces, mostly from U.S. Navy personnel, from 2004 to 2021. No details are given in the preliminary assessment. The report found that the UAPTF was unable to identify 143 of these objects, but it included five categories of potential explanations; "Airborne Clutter" includes objects like birds and balloons, "Natural Atmospheric Phenomena" includes atmospheric effect like ice crystals, "USG or Industry Developmental Programs" includes US military technology, "Foreign Adversary Systems" includes technologies developed by foreign governments such as Russia or China, and "Other", described as a "catchall" for other explanations. In contrast, much more conservative numbers for the percentage of UFOs were arrived at individually by astronomer Allan Hendry, who was the chief investigator for the Center for UFO Studies (CUFOS). CUFOS was founded by astronomer J. Allen Hynek (who had been a consultant for the Air Force's Project Blue Book) to provide a serious scientific investigation into UFOs. Hendry spent 15 months personally investigating 1,307 UFO reports. In 1979, Hendry published his conclusions in The UFO Handbook: A Guide to Investigating, Evaluating, and Reporting UFO Sightings. Hendry admitted that he would like to find evidence for extraterrestrials but noted that the vast majority of cases had prosaic explanations. He found 89% of reports definitely or probably identifiable and only 9% unidentified. "Hardcore" cases—well-documented events which defied any conceivable conventional explanation—made up only 1.5% of the reports.

Project Sign

Project Sign lists that "in order to investigate the credibility of their existence the following factors must be considered in any technical analysis":<ref>Project SIGN</rorce)</ref>

Method of support (lift)

Wings Fuselage lift (Wingless) Rotor Vertical Jet Magnus effect Aerostatic (lighter-than-air craft)

Method of propulsion (thrust)

Propeller reciprocating engine combination jet Rocket Ramjet Aerodynamic (for example, the Katzmayer effect, defined as "the reduction in drag of an airfoil when an air stream is oscillating").

Project Blue Book Special Report No. 14

Project Blue Book Special Report No. 14 was compiled between 1951 and 1954, and included 3201 reported UFO sightings. Battelle employed four scientific analysts, who sought to divide cases into "knowns", "unknowns", and a third category of "insufficient information." They also broke down knowns and unknowns into four categories of quality, from excellent to poor. In order for a case to be deemed "identified", two analysts had to independently agree on a solution and for a case to be called "unidentified", all four analysts had to agree. A report classified as "unidentified" was defined as: "Those reports of sightings wherein the description of the object and its maneuvers could not be fitted to the pattern of any known object or phenomenon."

Out of 3,201 cases, 69% were judged to be identified, 22% were unidentified, and 9% had insufficient information to make a determination.

Breakdown by category of IFO and case quality

Category/Case QualityAllExcellentGoodDoubtfulPoorAstronomical 22% 24% 23% 19% 23%Aircraft 22% 19% 22% 25% 16%Balloon 15% 12% 17% 17% 13%Light phenomena 2.2% .9% 2.4% 2.9% 1.1%Birds 1.0% 0.9% 1.0% 1.2% 0.7%Clouds, dust, etc. 0.4% 0% 1.0% 0.4% 0%Psychological 2.0% 0% 0.5% 3.3% 3.3%Other 5% 5% 5% 5% 6%Insufficient information 9% 4% 4% 14% 21%Unknown origin 22% 33% 25% 13% 17%

BBSR further broke these results down based on whether the identification was considered certain or merely doubtful. For example, in both the astronomical and aircraft IFO categories, 12% were considered certain and 9% were doubtful. Overall, of the 69% listed as IFOs, 42% were thought to be solved with certainty, while 27% were still considered doubtful.

In addition, if a case was lacking in adequate data, it was placed in the insufficient information category, separate from both IFOs and UFOs.

Military vs. civilian breakdown

IFO UFO Insufficient Information Mil Civ All Mil Civ All Mil Civ All Best reports 65% 72% 68% 32% 24% 28% 2% 4% 3%Worst reports 70% 70% 70% 24% 14% 16% 7% 17% 14%

The Battelle BBSR study included many internal military reports; 38% of the cases were designated as military. Military witnesses tended to submit better quality reports, had much fewer reports rated as having insufficient information, and had higher percentages of unknowns. As in the previous breakdown, the percentage of UFOs again rose with case quality for both the military and civilian subcategories.

In the summary table, best reports are those rated excellent and good; worst reports are doubtful and poor.

Comparison of IFOs to UFOs by characteristics A key study of BBSR was to statistically compare IFOs and UFOs by six characteristics: color, number of objects, shape, duration of observations, speed, and light brightness. If there were no significant differences, the two classes were probably the same, the UFOs then representing merely a failure to properly identify prosaic phenomena that could already account for the IFOs. On the other hand, if the differences were statistically significant, this would suggest IFOs and UFOs were indeed distinctly different phenomena.

In the initial results, all characteristics except brightness tested significant at less or much less than 1% (brightness was greater than 5%). By removing "astronomical" sightings from the "knowns" and redoing the test, just two categories, number and speed, were significant at less than 1%, the remainder having results between 3% and 5%. This indicated that there was a statistically significant difference between the characteristics ascribed to UFOs and IFOs, but perhaps not as significant as the initial results suggested. For two characteristics, brightness and speed, the significance actually increased with the revised test.

Allan Hendry study Like the Air Force, astronomer Allan Hendry found that only a small percentage of cases were hoaxes and that most sightings were actually honest misidentifications of prosaic phenomena. Hendry attributed most of these to inexperience or misperception.

Out of 1,307 cases Hendry deemed 88.6% had clear prosaic explanations (IFOs) and only 8.6% were unknowns (UFOs). Of the UFOs, Hendry reported that 7.1%, might still have a prosaic explanation while 1.5% (20 cases) had no possible plausible explanation and were completely unexplained. The remaining miscellaneous cases (2.8%) were "garbage" cases, where Hendry deemed the witnesses unreliable, the reports hopelessly contradictory, or lacking in sufficient information.

Overall, in the three major categories, 42% of all cases had astronomical explanations, 37% were aircraft, and 5% were balloons. A further breakdown allowed 77% to be readily explained by five main classes of objects: 29% were bright stars or planets, 19% were advertising planes, 15% were other aircraft, 9% were meteors and reentering space debris, and 5% were balloons of various types (mostly weather or advertising balloons but also a few prank balloons).

Breakdown of cases Hendry also used a case classification system developed by his mentor J. Allen Hynek, who established CUFOS, where the study was carried out. In this summary table: NL = "Nocturnal Lights", lights seen in the sky at night. DD = "Daylight Discs", objects seen in daytime (but not necessarily disc in shape). RV = "Radar/Visual" cases, objects observed by both witnesses and radar. CE = "Close Encounter" cases. For convenience, CE cases listed below are combined totals of Hynek's CE1, CE2, and CE3 cases, where: CE1 cases where objects were thought to be seen up close (within 500 feet). CE2 had purported physical interactions with the environment (physical trace cases or electromagnetic interference). CE3 cases were supposed to involve sightings of occupants.

CategoryNLDDCERVTotal CasesPercentAstronomical bright stars or planets 360 2 16 2 380 29%meteors, re-entering man-made spacecraft 113 5 4 0 122 9%artificial satellites 24 0 0 0 24 2%moon 22 0 0 0 22 2%TOTAL (all cases) 519 7 22 2 550 42%Aircraft advertising planes 230 0 22 0 252 19%other aircraft 196 22 6 0 224 17%missile launches 9 0 1 0 10 0.7%TOTAL 435 22 29 0 486 37%balloons 23 35 2 3 63 5%birds 5 1 0 0 6 0.5%clouds, dust 10 2 0 0 12 0.9%light phenomena (mirage, moon dog, ground lights, searchlights, etc.) 9 1 4 0 14 1.1%other (kites, flares, reflections, windblown debris, etc.) 12 3 1 0 16 1.2%Total Identified Cases 1024 71 58 5 1158 88.6%Percent 78.3% 5.4% 4.4% .4% 88.6% Total Unidentified Cases 79 18 16 0 113 8.6%Percent 6% 1.4% 1.2% 0% 8.6% MISC (insufficient information, inconsistent accounts, unreliable witnesses) 36 2.8%Total all cases Cases 1103 89 74 5 1307 100%Percent 84.4% 6.8% 5.7% 0.4% 100%

Common causes of misidentification and UFOs