condition. Once the condition has come to pass, it is not of further interest in survival analysis. The genetic effects which produce the condition are only significant at the onset of the condition, and their effects persist only if the subsequent onset of the condition in other subjects is attributable to them. In the twin studies post-onset presence of the condition is part of the outcome analyzed. That is, in the longitudinal studies using multistage models, the affected subjects are retained throughout the study subsequent to their becoming affected, while in the survival analysis method used in this study, the affected subjects are removed from consideration in the study once they have become affected, and no longer influence the results. Therefore, although the use of a longitudinal multi-stage model in van Beek et al. (2012) and Baker et al. (2011) enables genetic influences to have age-specific characteristics, these effects are modeled as persisting through time as a result of an effect at a single age range.
