Difference in differences “difference in differences” en.wikipedia.org/wiki/Difference_in_differences difference-in-differences difference-in-differences approach difference in differences differences-in-differences difference-in-differences difference in differences method Difference in differences estimation difference-in-differences Difference in differences difference in differences “difference in differences” view article Difference-in-Differences difference in differences method en.wikipedia.org/wiki/Difference_in_differences difference in differences English difference in differences