There were significant correlations between age and both DV (r = 0.366) and PPD (0.485). Therefore, prior to model-fitting analyses, both DV and PPD scores were regressed on age and age2 within each sex separately, and the residual scores were standardized. Because the scores were positively skewed, they were then log-transformed to approximate a normal distribution using the equation (y) = ln(2+x), where x is the original age corrected, standardized score, and (y) is the transformed score. The resulting scores were approximately normally distributed (DV: skew = 0.776, kurtosis = 2.812; PPD: = skew = -0.598, kurtosis = 2.180).
