One of the easiest ways to find out what OBE is like is to collect a large number of accounts of cases and compare them. In this way any common features can be extricated and variations noted. A great deal can be learned about the conditions under which the experiences occurred, how long they lasted, and what they were like. Accounts by people who have had OBEs fall, roughly speaking, into two categories.
here are the many ordinary people to whom an OBE occurs just once, or a few times; and there is a small number of people who claim to be able to project at will. The limitations of this method are that there are many important questions which cannot be answered by collecting cases. Since the people voluntarily report their experiences, the sample necessarily ends up with a bias. Many accounts are given many years or even decades after the event and it is then impossible to determine how much of the story has altered in memory with the passage of time. For such reasons it is not possible to determine, for example, how common the the experience actually is. Second, many OBEers claim that they were able to see rooms into which they had never been, describe accurately people they had never met, or move physical objects during their experience. Such reports are of great interest to parapsychology but they cannot be tested by collecting cases.