Before revision: Around 20\% of all empirical articles published by the American Economic Review between 2010 and 2012 estimate treatment effects using linear regressions with time and group fixed effects . In a model where the effect of the treatment is constant across groups and over time, such two-way fixed effects regressions identify the treatment effect of interest under the standard "common trends" assumption.
After revision: Around 20\% of all empirical articles published by the American Economic Review between 2010 and 2012 estimate treatment effects using linear regressions with time and group fixed effects are widely used to estimate the effect of a treatment on an outcome. If the effect of the treatment is constant across groups and over time, such two-way fixed effects regressions identify the treatment effect of interest under the standard "common trends" assumption.