Numerous publications on the UFO subject contain accumulated first-, second- and third-hand reports of UFO sightings. We propose that such reports be subject to a classification system that recognizes how well each case in question has been investigated. The classification is first applied in a German UFO database, which also contains numerous sighting reports from various sources. In this way, pure witness reports can be distinguished from extensively investigated UFO experiences, and the classification can be used to counter the argument that case collections combine heterogeneous statements about UFO sightings according to their origin and thus lose their value.