Quantitative methods and methodologies have dominated research on the efficacy and effectiveness of psychotherapy. The value of the quantitative paradigm is reflected in the criteria used to establish empirically supported treatments. However, quantitative research, including randomised control trials (RCTs), may not be ideally suited to establishing the effectiveness of the complex process of psychotherapy. The process and outcomes of psychotherapy have been regarded as causally entangled; the client's and therapist's efforts to responsively regulate the therapeutic process should be seen as being integral to outcomes rather than a source of confounding variance. Qualitative research may provide additional insights into the process of how psychotherapy is effective, due to its ability to explore phenomena from multiple perspectives. This is particularly important as research has suggested that qualitative differences in treatments can be masked beneath quantitatively equivalent outcomes. As a result, a continued overreliance on quantitative research may limit the discipline's overall ability to account for and differentiate the effectiveness of psychotherapies. This article proposes that, in order to address this limitation, (i) the criteria for how psychotherapies are considered effective should be expanded to include factors such as the efficiency of treatments, rates of change, and reliability of change; (ii) and that qualitative research be used adjunctively to assess and explore the dimensions of the expanded criteria. The development of a methodologically integrative multidimensional assessment of treatment effectiveness will provide a more informative tool to guide clinical decision making and policy.