Philip Klass is a noted skeptic of the idea that UFOs are extraterrestrial craft. In his books, UFOs Explained (1974) and UFOs: The Public Deceived (1997) he discusses 10 principles of investigating reports of UFOs. More information and examples of the principles (which are based on Klass's extensive experience investigating UFOs) can be found on the following web page, from which I have extracted the 10 principles. http://www.astronomyufo.com/UFO/Klass.htm Here I merely reproduce the principles, which are well worth paying attention to when investigating any reported UFO. 1. Basically honest and intelligent persons who are suddenly exposed to a brief, unexpected event, especially one that involves an unfamiliar object, may be grossly inaccurate in trying to describe precisely what they have seen. 2. Despite the intrinsic limitations of human perception when exposed to brief, unexpected and unusual events, some details recalled by the observer may be reasonably accurate. The problem facing the UFO investigator is to try to distinguish between those details that are accurate and those that are grossly inaccurate. This may be impossible until the true identity of the UFO can be determined, so that in some cases this poses an insoluble problem. 3. If a person observing an unusual or unfamiliar object concludes that it is probably a spaceship from another world, he can readily adduce that the object is reacting to his presence or actions when in reality there is absolutely no cause-effect relationship. 4. News media that give great prominence to a UFO report when it is first received, subsequently devote little if any space or time to reporting a prosaic explanation for the case when all the facts are uncovered. 5. No human observer, including experienced flight crews, can accurately estimate either the distance/altitude or the size of an unfamiliar object in the sky, unless it is in very close proximity to a familiar object whose size or altitude is known. 6. Once news media coverage leads the public to believe that UFOs may be in the vicinity, there are numerous natural and man-made objects which, especially when seen at night, can take on unusual characteristics in the minds of hopeful viewers. Their UFO reports in turn add to the mass excitement which encourages still more observers to watch for UFOs. This situation feeds upon itself until such time as the news media lose interest in the subject, and then the "flap" quickly runs out of steam. 7. In attempting to determine whether a UFO report is a hoax, an investigator should rely on physical evidence, or the lack of it where evidence should exist, and should not depend on character endorsements of the principals involved. 8. The inability of even experienced investigators to fully and positively explain a UFO report for lack of sufficient information, even after a rigorous effort, does not really provide evidence to support the hypothesis that spaceships from other worlds are visiting the Earth. 9. Whenever a light is sighted in the night skies that is believed to be a UFO and this is reported to a radar operator, who is asked to search his scope for an unknown target, almost invariably an "unknown" target will be found. Conversely, if an unusual target is spotted on a radarscope at night that is suspected of being a UFO, an observer is dispatched or asked to search for a light in the night sky, almost invariably a visual sighting will be made. 10. Many UFO cases seem puzzling and unexplainable simply because case investigators have failed to devote a sufficiently rigorous effort to the investigation.