In addition, the interaction between SNP effects and age (infancy vs. puberty) on peak height velocity (PHV) was tested. This was necessary as especially in the context of low power; finding that some SNPs are statistically significantly associated with PHV at one age and not the other does not automatically indicate different pattern of associations between these ages. Since PHV is much higher in infancy than in puberty, PHV Z-scores were calculated from the log-transformed PHV variables at each age to unify their scale. The data from infancy and puberty were combined into a single data set where most individuals had PHV values for both ages, i.e. two records per person, age indicator variable referring to the time when PHV was estimated (0 = infancy, 1 = puberty). A mixed model for repeated measures that takes into account the within-person correlation in the outcome values was chosen. The mixed model was fitted between each SNP and PHV Z-score without pre-defined covariance structure for the error matrix (type = unstructured), with SAS PROC MIXED (version 9.1.3.). Age was included into the
