Understanding how and why adding valid predictors can decrease the validity of selection composites: A generalization of Sackett, Dahlke, Shewach, and Kuncel (2017)

It is usually assumed that adding more valid predictors will increase the predictive power of a selection test battery. Sackett, Dahlke, Shewach, and Kuncel showed that when selection tests are combined using unit weights, adding a valid predictor can lead to a decrease in validity. Situating the Sackett et al. approach in a more general multivariate framework I show how: (a) it is the tradeoff between predictor validity and predictor intercorrelations, and not the differences in predictor validities that determines whether adding a valid predictor to a composite will cause the validity of that composite to increase or decrease; and (b) this same dynamic applies across a wide range of non-optimal schemes for weighting predictors and/or criteria.