Window variables are the variables measured during the constrained interval of an observation period which is accepted as the proxies for the information over the whole period. For instance, many statistical studies of the determinants of the children's attainments measure the circumstances or the events occurring over the childhood period by the observations of these variables for a single year or a short duration of during childhood. The parameter estimates based on such variables will often be biased and inconsistent estimates of the true underlying relationships, relative to variables which reflect circumstances or events over the whole period.