On Cronbach's merger: Why experiments may not be suitable for measuring individual differences
We investigate Cronbach's 1956 call to merge differential and experimental psychology by using true experiments to study individual differences. Through simulation, we show that experimentally-defined contrasts are too noise-prone to be useful at the individual level, making it difficult to uncover even simple latent structures. We introduce a new signal-to-noise ratio measure of task goodness that is invariant across experiments. Our findings reveal that latent cluster or factor structures were only recoverable in the largest experiments (hundreds of people, hundreds of trials per condition), and only for the simplest structures. These results serve as a warning: while Cronbach's merger is theoretically appealing, it faces substantial practical hurdles.