It must be remembered that, after careful investigation, over 90% of UFO reports can be reasonably explained as manmade or natural phenomena.
The late Charles H. Gibbs-Smith (aviation historian to the Victoria and Albert Museum in London) had something he called Gibbs-Smith’s rule which is worth keeping in mind when studying UFO reports.
It states that “the strangeness of a case increases in proportion to the distance, in either time or geographical distance, between the investigator and the location of the report.”
The main classification of UFO reports in use is based upon one used by Dr J. Allen Hynek in his book “The UFO Experience” (Aberlard-Schuman 1972).
Hynek was the man behind most of the early government projects: Grudge, Sign and BlueBook and his book was based on the hundreds of cases he personally investigated.
“The witnesses I interviewed could have been lying, could have been insane or could have been hallucinating collectively—but I do not think so,” …. Their standing in the community, their lack of motive for perpetration of a hoax, their own puzzlement at the turn of events they believe they witnessed, and often their great reluctance to speak of the experience—all lend a subjective reality to their UFO experience.”
It should be noted that many other classification systems have been devised by other researchers. Briefly the Hynek system (with the most commonly used extensions) is :
LONG RANGE SIGHTINGS
Further than 500ft. distant from UFO
NL (nocturnal light)
A simple visual sighting of a unidentified flying light seen at night.
This group contains 35 to 40 percent of all UFO reports.
ND (nocturnal disc)
A simple visual sighting of a unidentified flying extended or structured
light source seen at night. (This is an extension to Hynek’s system).
DD (daylight disc)
A simple visual sighting of a UFO with distinct shape seen during the day.
RV (Radar Visual Cases)
UFOs observed visually whilst being simultaneously on radar. From Hynek’s
study these make up 1 to 2% of reports.