The Hauntings of Everyday Life states that difficulty in scientifically evaluating ghosts is that a surprisingly wide variety of phenomena are attributed to ghosts, from a door closing on its own, to missing keys, to a cold area in a hallway, to a vision of a dead relative.
When sociologists Dennis and Michele Waskul interviewed ghost experiences they found that many were uncertain if they had encountered a ghost. Instead, many of the respondents were simply convinced that they had experienced something uncanny ~ something inexplicable, extraordinary, mysterious, or eerie.
Thus, many people who go on record as claiming to have had a ghostly encounter didn’t necessarily see anything that most people would recognize as a classic ‘ghost’ and in fact they may have had completely different experiences whose only common factor is that it could not be readily explained.