One paranormal researcher is of the opinion that " a vast porportion of reports, possibly as many as 98%, can be solved by identifying the 'haunting' as something quite mundane."
Then he goes on to say "Of course, the other two percent can be absolutely fascinating."
Fascinating because the facts as presented are resistant to the usual grounds for dismissal -- illusion, delusion, confusion or deliberate fraud.
Researchers classify ghostly phenonema according to various criteria. One type is referred to as a crisis apparition -- ghosts of the dying.
Several famous cases of a clear, substantial apparition visiting a close friend or relative with the news -- before that relative could have received the confirmation from a conventional source (because of distance) -- resist explanation by the laws of physics as we know them.
Further, a number of the more seriously regarded cases were not based on the testimony of a single individual. Apparations were observed and recognized by several people.
Attemps to elucidate the mechanism for this type of appparition often involve unstable and complex theoretical extensions such as telepathy, bi-location, out-of-body experience, or survival of the spirit, ie. illustrating an unknown by an unknown.
Probity rests largely on the fact that the recipient of such visions could not have known of the precipitating event because normal communication was either absent or unavailable -- how could a mother see her son in England if he's presently on a ship going down in the south Atlantic. So one wonders if -- in this age where contact is available 27/7 -- claims of such visitations will substantially diminish.