under fixed effects estimation, we assume that the un-observed effect is fixed over time. The time variation between independent and dependent variables are only in each cross-sectional
observation.418 By this method, the constant unobserved effect over time is not now a problem, and we do not need to
assume that the unobserved effect is uncorrelated with the independent variables.419 We also suppose that the error
terms are homoskedastic and serially uncorrelated (across the time). On the other hand, by random effect estimation,
we suppose that the unobserved effect is uncorrelated with the independent variables in all time periods.