Number of participants required for common designs in psychology: A power analysis

Given that the average effect size of pairwise comparisons in psychology is d = .4, very few studies are properly powered with less than 50 participants. For most designs and analyses, numbers of 100, 200, and even more are needed. These numbers become feasible with the recent introduction of internet-based studies and experiments, although they also require a change in the way research is evaluated by supervisors, examiners, reviewers, and editors. The present paper describes the numbers needed for the designs most often used by psychologists, including single-variable between-groups and repeated-measures designs with two and three levels, two-factor designs involving two repeated-measures variables or one between-groups variable and one repeated-measures variable (split-plot design). The numbers are given for the traditional, frequentist analysis with p 10. These numbers should give a straightforward answer to researchers asking the question: “How many participants do I have to include in my experiment?” We also discuss how researchers can improve the power of their study by including multiple observations per condition per participant.