Provisional visual inspection of individual growth plots of P300 amplitude by age suggested a linear model of change for this age range (Carlson & Iacono, 2006). SAS PROC MIXED (SAS Institute, 1998) with full maximum likelihood estimation, unstructured residual variance matrices, and the “between– within” method for determining degrees of freedom was used to fit mixed models as suggested by Singer and Willet (2003). SAS PROC MIXED assumes that data are missing at random. The models fit to P300 amplitude were “three-level” linear mixed models appropriate for use with multiple participants from the same family (Khoo & Muthén, 2000). As such, random effects were allowed for intercept, slope, and their covariance at the level of both the individual participant and the family.
