Fixed Effects Estimator

Requires strict exogeneity $$E[u_{it}|x_{i},c_{i}]=0 \, t=1,...T$$. Intuitively, this is because every $$y_{it}, x_{it}$$ needs to be demeaned (the within transformation) - since the mean depends on past, current and future values, we need that $$u_{it}$$ is uncorrelated with all $$t$$.