The chapter provides the two main research formats used to collect data in psychology, the experimental and the correlational study. The correlational approach is at the heart of psychometrics—the measurement and theory of individual differences. The simple correlational study is concerned with measuring the strength of association between two variables. The researcher simply passively observes the relationship between a pair or pairs of variables. Correlational studies allow analysis of behaviour that has been passively observed—rather than collected under experimentally rigorously controlled conditions. In a within-subject or repeated measures experiment it is possible to report a measure of the correlation in performance for the subjects in the control and experimental conditions. Experiments are just one of the methods for attempting to make inferences about causal relationships between variables. There is also a method that can be applied to data collected by passive observation rather than by active experimentation.