If you have panel data, your econometric model can explicitly estimate the unobserved effects associated with your cross-sectional unit using the fixed effects (FE) model: Yit =
β0 + β1Xit + β2wit + εit, where wit = wi are unobserved
characteristics for each cross-sectional unit that don’t vary
over time. (I explain how to estimate this model in the
preceding section.) On the other hand, your econometric
model can allow all unobserved effects to be relegated to the
error term by specifying the model as