Individual differences methods for randomized experiments

Experiments allow researchers to randomly vary the key manipulation, the instruments of measurement, and the sequences of the measurements and manipulations across participants. To date, however, the advantages of randomized experiments to manipulate both the aspects of interest and the aspects that threaten internal validity have been primarily used to make inferences about the average causal effect of the experimental manipulation. This article introduces a general framework for analyzing experimental data to make inferences about individual differences in causal effects. Approaches to analyzing the data produced by a number of classical designs and 2 more novel designs are discussed. Simulations highlight the strengths and weaknesses of the data produced by each design with respect to internal validity. Results indicate that, although the data produced by standard designs can be used to produce accurate estimates of average causal effects of experimental manipulations, more elaborate designs are often necessary for accurate inferences with respect to individual differences in causal effects. The methods described here can be diversely applied by researchers interested in determining the extent to which individuals respond differentially to an experimental manipulation or treatment and how differential responsiveness relates to individual participant characteristics.