Falsifications are just as important as event studies

Some personal comments about parallel trends, no anticipation and falsifications

Today’s difference-in-differences is not our grandfather’s difference-in-differences. We have survived a war but we lived to tell. We learned that our greatest sin had the unthinkable comparison of treated groups to already-treated groups. We repented, and swore it would never happen again.

But as the dust cleared, and thousands of citations accumulated …