Issues and findings in investigating predictors of psychotherapy outcome: Introduction to the special section

Over the last two decades, much ink has beenspilled in the debate over the relative efficacy ofdifferent therapies. Many researchers have tried,with some success, to demonstrate that sometherapies are more effective than others, whereasother researchers have argued that all psychothera-pies are equivalent. Underlying this controversy isthe realization that much is at stake in determiningwhether therapies differ in effectiveness. The topicof this special section is loosely related to thisenduring debate, but it is built on the fact thateven if patients in different treatments do equallywell on average, there is always a range in patientoutcome. This being the case, no single therapy islikely to be most efficacious in all cases, and atherapy with a high success rate among certainsubtypes of patients and in certain situations maynot be as effective in another context. The ques-tion of which therapy is best has to be supple-mented by the question, ‘‘Which factors contributeto positive outcomes in each type of therapy, indifferent therapies, and across therapies? ’’ Under-standing the predictors that are responsible for therange of patient outcome in specific treatmentsmay enable us to recognize the most suitabletreatment for each patient (e.g., Beutler, 1991). This special section is devoted to studies that havefurthered our understanding of this crucial andvaried relation between predictors and outcomes inspecific therapeutic contexts.Many terms have been used in the literature todescribe the predictive relation between variousvariables and outcome. We use the most generic,general, and inclusive of these terms, ‘‘predictors ofoutcome,’’ in the title of this special section. Morespecific terms have been used with a similar,although not identical, intention. ‘‘Aptitude bytreatment interaction’’ (ATI; e.g., Beutler, 1991)and ‘‘moderators of outcome’’ are similar in thatboth refer to baseline or pretreatment variables thatpredict outcome. The former seems to have been aterm introduced in education (Dianne Chambless,personal communication, October 17, 2006),whereas the latter was coined in the more formalwritings of Baron and Kenny (1986) and Kraemer,Wilson, Fairburn, and Agras (2002). Because moderation and ATI research are aboutdemonstrating that one variable has a differentialimpact within two different treatments, if one looksat the predictive value of one variable in onetreatment group, then one cannot speak aboutmoderation per se but only about prediction. Simi-larly, if one examines the predictive power of avariable across groups (i.e., main effects) withoutexamining the interaction of the predictor withtreatment, then one is studying general predictorsof outcome rather than moderation. In sum, pre-dictor of outcome is a more general class thanmoderators.Regardless of the label, all authors agree about thecentral formal requirements involved in testing formoderation. Namely, a statistically significant inter-action must be demonstrated between a pretreat-ment variable and treatment group in predictingsome outcome variable. Thus, if a variable is foundto be a moderating variable, its assessment beforetreatment can help us determine which treatment isbetter for a subset of the participants. Such researchclearly has great clinical and theoretical importance.For example, Elkin et al. (1989) have shown thatpatients receiving cognitive therapy (CT) or inter-personal therapy (IPT) had similar outcomes in the