How often is prep close to the true replication probability?

Largely due to dissatisfaction with the standard null hypothesis significance testing procedure, researchers have begun to consider alternatives. For example, Killeen (2005a) has argued that researchers should calculate prep that is purported to indicate the probability that, if the experiment in question were replicated, the obtained finding would be in the same direction as the original finding. However, Killeen also seems to indicate that rather than being the probability of replication, prep is actually the probability of obtaining a finding whereby the experimental group mean exceeds the control group mean. Our goal was to determine the relative frequency with which obtained prep statistics are close to true replication probabilities. Regardless of which way prep is defined, our simulations show that it is unlikely to be close to the true value unless both the population effect magnitude and the sample size are uncommonly large. The definitional problem in combination with the inaccuracy under either interpretation, constitutes an important challenge for those who espouse the routine computation of prep statistics.