Adjusting researchers’ approach to adjustment: On the use of covariates when testing interactions

A common design in social psychology involves the use of two independent variables, an experimental manipulation and a measured individual difference, and the interest is in the interaction between them. In such designs, there are often obvious covariate(s), correlated with the measured independent variable, which the researcher wishes to control. Typically this is done by including the covariate in the analytic model. We show that in most cases this is an inadequate model. In general, the interaction between the two independent variables will be estimated without bias only when the interaction between the covariate and the manipulated independent variable is included in the analysis. We present simulations showing the factors affecting the magnitude of the bias and provide a survey of recent social psychological literature illustrating the frequency of the problem. Finally, we discuss cases where both independent variables are manipulated and the covariate is a potential mediator.