Case sampling for psychotherapy practice, theory, and policy guidance: Qualities and quantities.

Random sampling of cases is usually infeasible for psychotherapy research, so opportunistic and purposive sampling must be used instead. Such sampling does not justify generalizations from sample to population-distribution statistics, but does justify reporting what independent-variable value configurations are associated with what dependent-variable value configurations. This allows only the generalization that these associations occur at least that frequently in the population sampled from, which is enough for suggesting and testing some psychotherapy theories and informing some psychotherapy practice. Although psychotherapy practice is a longitudinal process, formal psychotherapy outcome research is so far most feasible and most widely done in the form of two-phase cross-sectional input-outcome studies. Thus, the analysis of sampling for psychotherapy research here will be in terms of the independent- and dependent-variable value configurations produced in such two-phase studies.