Quantitative psychotherapy outcome research: Methodological issues

Abstract

This chapter reviews methodological issues in quantitative psychotherapy outcome research. In the era of evidence-based medicine, it is vital for researchers to understand the methodological details that ensure the validity of outcome studies. By re-examining some “classical” examples of psychotherapy outcome studies, this chapter explains the relative strengths and merits of different research designs. It discusses why control groups are needed; why large sample sizes are crucial but cannot replace other design aspects; why designs can be ranked according to their internal validity; and how internal validity can be ensured without neglecting external validity. Both efficacy and effectiveness studies can use randomised controlled trial designs; however, the relevance of all design aspects to usual practice is emphasised more in effectiveness trials. This concerns participants, interventions, control conditions and outcomes. Interventions are often applied more flexibly in effectiveness trials. This chapter places most emphasis on design aspects because, in contrast to statistical analyses, they are more basic; they cannot be corrected after completion; and they are a precondition for a correct analysis. However, some basic principles for statistical analysis, as well as common pitfalls, are also explained briefly. In summary, this chapter should enable the reader to make informed decisions to design outcome studies that are clinically relevant and methodologically sound.