![](https://www.andrewheiss.com/blog/2021/08/25/twfe-diagnostics/index_files/figure-html/show-weights-hist-1.png)
The world of econometrics has been roiled over the past couple years with a bunch of new papers showing how two-way fixed effects (TWFE; situations with nested levels of observations, like country-year, state-month, etc.) estimates of causal effects from difference-in-differences-based natural experiments can be biased when treatment is applied at different times.