Less Is More: Psychologists Can Learn More by Studying Fewer People

Psychology has been embroiled in a professional crisis as of late. The research methods commonly used by psychologists, especially the statistical analyses used to analyze experimental data, are under scrutiny. The lack of reproducible research findings in psychology and the paucity of published studies attempting to replicate psychology studies have been widely reported (e.g., Pashler and Wagenmakers, 2012; Ioannidis et al., 2014; Open Science Collaboration, 2015). Although it is encouraging that people are aware of problems evident in mainstream psychology research and taking actions to correct them (e.g., Open Science Collaboration, 2015), one problem has received little or no attention: the reliance on between-subjects research designs. The reliance on group comparisons is arguably the most fundamental problem at hand because such designs are what often necessitate the kinds of statistical analyses that have led to psychology’s professional crisis (Sidman, 1960; Michael, 1974; Parsonson and Baer, 1978). But there is an alternative. Single-case designs involve the intensive study of individual subjects using repeated measures of performance, with each subject exposed to the independent variable(s) and each subject serving as their own control (Sidman, 1960; Barlow et al., 2008; Johnston and Pennypacker, 2009; Kazdin, 2010). Comparisons of performance under baseline and experimental conditions are made for each subject, with any experimental effects replicated with the individual subject across time or across multiple subjects in the same experiment. Single-case experiments yield data that can be interpreted using non-inferential statistics and visual analysis of graphed data, a strategy characteristic of other natural sciences (Best et al., 2001). Single-case experimental designs are advantageous because they more readily permit the intensive investigation of each subject and they achieve replication within an experiment rather than across experiments. Thus, data from just a few subjects tells a story.