East Carolina University
Department of Psychology
We thank Ray Koopman for noticing that there was a problem
with the original version of our t-test for comparing two independent
ordinary least squares (OLS) regression coefficients. Ray also noticed that we
had not implemented Steiger's (1980) adjustment when computing the standard
errors for the PF and ZPF tests. An Errata document is forthcoming, and should
appear on the journal website soon. Meanwhile, some details are provided below.
Problem with t-test for comparing two OLS regression coefficients
We computed the standard error of the difference between the two coefficients using a method that does not assume equal variances. Therefore, we ought to have used Satterthwaite degrees of freedom (df), as is done when using the unequal variances version of the t-test for comparing two means. We have modified our code to use the correct df for that t-test. Our revised code also computes the pooled variance version of the same t-test. Users can indicate which version of the test they want by setting input variable Pool = 1 (for the pooled variance test) or Pool = 0 (for the unequal variances test). Note that the pooled variance test is the one that corresponds to Potthoff analysis, which can be carried out if one has the raw data.
Steiger's adjustment when computing PF and ZPF
Steiger's adjustment consists of replacing both r12 and r34 with the mean of r12 and r34 in the equations for their respective standard errors, including the computation of k (see equations 18 and 19 in the original article). Therefore, we have also modified our code for PF and ZPF to compute both Steiger's modified versions and the original versions of PF and ZPF. Users can indicate which one they want by setting input variable Steiger = 1 (for Steiger's modified versions) or Steiger = 0 (for the original versions).