The programme's unit of evidence is a documented case rather than a trial, and understanding what a case consists of is essential to weighing the claim. A typical file begins with a young child, usually between two and seven, who has begun making statements about another life: a name, a place, a family, an occupation, sometimes a manner of death. Ian Stevenson or a colleague interviews the child and the family, recording the statements as reported. Where possible they then attempt to trace a deceased person matching the description, and if one is found they interview that family too, comparing what the child said against the facts of the life.
Some files add a physical element: a birthmark or birth defect on the child said to correspond to a wound recorded in the previous person's postmortem, which Stevenson documented photographically and against medical records where he could obtain them. More than two thousand cases were accumulated over four decades, drawn heavily from cultures where reincarnation is an accepted belief. Stevenson's own language was consistently guarded, and his best known book is titled Twenty Cases Suggestive of Reincarnation. He held a chair in psychiatry at the University of Virginia throughout, and the programme was funded by a private bequest rather than by public research money.