Participants changed their behavior upon realizing the purpose of the study
Provider in a psychotherapy trial are aware of the investigators' hypotheses, and this knowledge unconsciously influence the way they rate participants' progress
Some participants leave in the middle of the study because of bad experiences, unwanted side effects, or inadequate incentives for participation, among other reasons.
Participants enrolled from a specific age group or those with mild symptoms may result in findings that are not generalizable to the broader population