y 0 1dB 2dE 3dB dE 0d2
1d2 dB 2d2 dE 3d2 dB dE u
The coefficient of interest is 3, the coefficient on the triple interaction term, d2 dB dE. The OLS
̂ estimate 3 can be expressed as follows:
3 ȳB,E,2 − ȳB,E,1 − ȳA,E,2 − ȳ
where the A subscript means the state not implementing the policy and the N subscript represents the non-elderly. This is the difference-in-difference-in-differences (DDD) estimate. ∙ Can add covariates to either the DD or DDD analysis to (hopefully) control for compositional changes. ∙ Can use multiple time periods and groups.