One writes about successful case histories in which treatments work, in which a theory is confirmed, or in which a rare disorder presents, but one does not usually write up treatment failures, the failures of a technique within a treatment, or tedious cases. In order for this database to be useful, the problems of selection for positive outcomes, selection for theory confirmation, selection for positive techniques within a treatment, selection for the dramatic, and selection for only the elements that confirm one's biases must be overcome. The database must include failure experiences as well as success experiences, blind alleys, alternative biases, and tedious cases. Related to the problem of selectivity is the problem of inferring etiology. Case histories are poor material for making causal inferences about etiology.
Case histories start with a patient who has a problem.
The patient then discusses his or her past history as it relates to this problem. It is tempting to use such a database to make inferences about the cause of the problem based on past history. One might, for example, collect a large number of cases of adult loneliness and depression that also have the reports of childhood sexual abuse. However, such a psychotherapy database fails to include individuals who do not have the presenting problem but who did suffer the conditions that correlate with a problem in the patients. So in this example, one would never see the many individuals who are depression-free and happily married who also suffered childhood sexual abuse.