Measures of independent variables and mediators are useful in social psychology experiments: but are they necessary?

A study of experiments in major social psychology journals shows that measures of independent variables have become increasingly common. The role in experiments of measures of independent variables and proposed mediating variables is examined. In the causal sequence assumed in interpreting an experimental result, the independent variable and proposed mediating variable are presumed to mediate the effect of the experimental treatment on the dependent measure. Measures of independent variables and mediators provide checks on the assumptions that the experimental treatment successfully manipulated those variables and are unquestionably useful. A separate, controversial issue is whether such measures are necessary in experiments. If no plausible alternative explanations exist, data from such measures are not needed. Plausible alternative explanations are not eliminated by data from such measures. Alternative explanations, critical for assessing construct validity (Cook & Campbell, 1979), are distinguished from different general theoretical accounts of a finding.