Incorporating time-in-sample in longitudinal survey models

At the time of recruitment, the participants in a longitudinal survey are chosen to be representative of a population. As time goes on, typically some of the participants will drop out, and dropout may be informative in the sense of depending on the response variables of interest. However, even if dropout is minimal, the participants who continue to the second and third waves of a longitudinal survey may differ from those they supposedly represent in subtle ways. It is clearly important to take such possibilities into account when designing and analyzing longitudinal survey data before and after an intervention.