difference in difference theory definition difference in time analysis definition why is a difference in difference technique used differences in time-series why is difference in difference used difference in differences in time series what is difference-in-difference difference in difference models what is the difference in difference? different difference analysis techniques difference in difference analysis what is difference in difference difference in difference analysis difference in difference models different in difference how does the difference in difference technique work why do ada and difference in differentiation analyses use intermittent time series what is difference-in-difference difference in difference definition how do difference-in-difference studies work?