In addition to accounting for significant variance at age 20 and 24, the SNP score also accounted for significant covariance (1.0%, p < .05, as tested by 200 bootstrap replications) between CPD at age 20 and 24, indicating that the SNP score is not only associated with CPD at 20 and 24, but also associated with stability in CPD for those individuals smoking between ages 20 and 24.