Clinical psychological science is improved when it seeks to understand not only whether an effect exists but also how that effect operates and its boundary conditions. Mediation and moderation analysis are widely used in clinical psychological research to explore and test hypotheses about the mechanisms by which causal effects operate and the contingencies of those effects. Their integration as conditional process analysis allows for the examination of the contingencies of those mechanisms - for whom or in what circumstances a particular mechanism is in operation or whether it is strong as opposed to weak. This chapter reviews the fundamentals of mediation, moderation, and conditional process analysis using ordinary least squares regression, commenting along the way on good practice as well as various misunderstandings in circulation. It illustrates the application of these fundamentals and their implementation using the PROCESS macro for SPSS and SAS.
