Mental chronometry and individual differences: modeling reliabilities and correlations of reaction time means and effect sizes.

We used a general stage-based model of reaction time (RT) to investigate the psychometric properties of mean RTs and experimental effect sizes (i.e., differences in mean RTs). Using the model, formulas were derived for the reliabilities of mean RTs and RT difference scores, and these formulas provide guidance about the number of trials per participant needed to obtain reliable estimates of these measures. In addition, formulas were derived for various different types of correlations computed in RT research (e.g., correlations between a mean RT and an external non-RT measure, between two mean RTs, between a mean RT and an RT effect size). The analysis revealed that observed RT-based correlations depend on many parameters of the underlying processes contributing to RT. We conclude that these correlations often fail to support the inferences drawn from them and that their proper interpretation is far more complex than is generally acknowledged.