As Biewen (2009) explains, these estimators may not work well in practice because estimation is based on histories with particular patterns and there are typically rather few of these. Usable histories are those with specific combinations of change in the outcome variable and (lack of) change in the explanatory variables over at least three consecutive periods. Inter alia, for A, and y to be identified, the explanatory variables need to change over time but not too quickly. Note also that an APE for A, cannot be calculated using the fixed effects approach. Another issue regarding specification of the basic DREP model is that genuine state dependence is assumed to be the same for all individuals: there is a single state dependence parameter, X, common to all individuals. Heterogeneity in state dependence can straightforwardly be introduced in three ways.