Longitudinal Analysis when the Experimenter does not Determine when Treatment Ends: What is Dose-Response?

The most appropriate amount of psychotherapy to address a particular problem is of interest to clinicians, consumers and those responsible for funding of care. The dose-response relationship has been examined within the context of randomized clinical trials, meta-analysis as well as naturalistic studies; however, each of these approaches has limits. Many of these approaches have conceptually blurred two distinct concepts: do participants with different characteristics need different amounts of therapy and do otherwise equal participants show different outcomes when given different levels of (a particular type of) therapy? For any study design, if the experimenter does not determine the duration of therapy, then the length of therapy is said to be endogenous. This endogeneity can bias any attempt to untangle the answer to these two questions. An extension of the biasing effect of this endogeneity involves the choice of times to assess outcome; if outcome assessment depends on when therapy is terminated (rather than exogenously chosen) then estimates of the trajectory of outcome may be biased. Design considerations to minimize this effect are discussed.