Δyit gi t Δwit Δxit Δuit.
Can difference again or estimate (17) by FE. ∙ Can derive standard panel data approaches using the counterfactural framework from the treatment effects literature. For each i, t, let yit1 and yit0 denote the counterfactual outcomes, and assume there are no covariates. Unconfoundedness, conditional on unobserved heterogeneity, can be stated as
Eyit0|wi, ci0, ci1 Eyit0|ci0 E y i t 1 | w i , c i 0 , c i 1 E y i t 1 | c i 1 ,
where wi wi1, . . . , wiT is the time sequence of all treatments. If the gain from treatment only depends on t,