Having dealt with (and read the reports of "ghost investigations" by) amateur ghost hunting groups from across the country, I have noticed an interesting logical sleight of hand that allows them to frame their investigations as successful even when they fail. Many, of course, take their cues from TV shows like Ghost Hunters and Paranormal State , featuring ordinary folks (plumbers, college students, musicians, housewives, etc.) who have little or no training in science, investigation, logic, or critical thinking. Thus it’s not surprising that their "investigations" invariably fail to find any real proof of ghosts, and have little to do with science or skepticism– no matter how often they refer to themselves as "skeptics."
Ghost hunters will wander around a reputedly haunted location with cameras, voice recorders, and other gadgets, looking for any "anomaly." There are three possible outcomes of this sort of pseudoinvestigation: 1) The ghost hunters will effectively debunk a particular phenomenon or claim (such as finding that a "mysterious" cold spot in a room is due to an unseen air conditioning vent); 2) The ghost hunters will not find any evidence of anything unusual at all (no "orbs" in photos, no "cold spots," no "anomalous" readings or faint ghost voices, etc.); or 3) The ghost hunters will find one or more phenomena that they cannot explain or understand (such as a strange sound, an EVP "voice," an orb or blur in a photograph, etc.). On the surface, there doesn’t seem to be any problem with this methodology— except that it’s logically and scientifically unsound, and leads to false-positive "evidence" for ghosts.
One of the hallmarks of good scientific methodology is that a claim, proposition, or hypothesis must be falsifiable; that is, there must be some way to determine whether an event occurred or it didn’t, a phenomenon exists or it doesn’t. If I claim that an invisible, undetectable polar bear is living in my garage, that may be true but is not a testable, falsifiable claim, because if an investigator searches for evidence and finds none at all, I can just say, "Of course you couldn’t find evidence for it, the polar bear is undetectable." Thus it is an untestable, scientifically worthless claim.
Many ghost hunters have managed to frame their "investigations" in exactly the same unscientific, untestable way. In the curious world of ghost hunting, the fact that the teams of ghost hunters cannot explain phenomenon is taken as a sign of their expertise. That is, they will confidently proclaim a location haunted if they could not find an explanation for some (apparently) strange phenomenon or other; they couldn’t figure it out, couldn’t solve the mystery.
The ghost hunters set up an interesting no-lose situation for themselves. If they are able to debunk or find ordinary explanations for ghostly phenomenon, then that shows what excellent investigators they are because they cleverly figured it out. On the other hand, if they can’t figure out an explanation for some phenomenon, then that also demonstrates what fantastic investigators they are, because they claim it is evidence of ghosts! And if they don’t find any evidence of ghosts, that of course does not prove that ghosts don’t exist, it can just be interpreted to mean that there were no ghosts active there at the time the ghost hunters were there; the door is left wide open for later investigations. Follow-up investigations are likely to find some "anomaly" or other (especially given the lax standards of evidence, since all that is needed to create evidence for ghosts is for one person to say, "I don’t really understand this"), thus virtually guaranteeing that many "haunted locations" will be deemed to have some "unexplained" activity, whether ghosts reside there or not.
Imagine if other investigative professions operated the same way, claiming success when they were unable to solve a problem: Police detectives unable to solve crimes would be promoted; doctors unable to correctly diagnose diseases would be congratulated and receive awards; mechanics who couldn’t explain and fix automotive problems would be successful. Usually, inability to accomplish a goal (such as explaining a mystery) is seen as an obvious failure; for ghost hunters, it’s a proud badge of success! If the purpose of investigation is to collect good scientific evidence for ghosts, the ghost hunter groups have completely failed.