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Agreed- much like fairy sightings, ghost sightings appear break down into several quite distinct 'types' of sightings, so 'vague shapes', solid representations of people, translucent figures). You'd probably be able to extract some interesting patterns to sightings by tabulating them, adding columns for features of the 'sighting' and also adding in stuff like weather, location, time of day, some info about the witness etc. etc.
Or astral travel. On another thread, I relayed the experience I had at 7 or 8 when I saw my youngest brother standing, clear as day in his pjs, at the side of my bed looking down at me early one morning. He might have been 1 1/2 to 2 years old because he was walking. This was not an uncommon thing for him to do as he'd join me and my sister in bed. I whispered his name, and he slowly disappeared. Was he astral projecting? I was not dreaming because I had woken and then dived right under my covers when he disappeared.