Objective: Whereas the fact of attrition during the course of treatment is well documented, little is known about the factors that affect sample selection before the beginning of a study ("pretreatment attrition"). The present study reports on the degree and sources of pretreatment attrition at two sites of a multicenter study on panic disorder that compared treatment outcomes for imipramine and cognitive behavior therapy.
Method: Data were collected at two clinical research sites, one with a pharmacological treatment orientation (N = 420) and one with a psychosocial treatment orientation (N = 208).
Results: The main source of pretreatment attrition was participant refusal. At both research sites, eligible patients most often refused participation because they were either unwilling to start treatment with imipramine (30.6% and 47.4%, respectively) or discontinue their current medication (22.6% and 35.1%, respectively).
Conclusions: Results from comparative treatment outcome studies are limited not only to people who meet the study criteria but also to those who are willing to begin a medication treatment and discontinue their current medication.