If you have panel data and believe that variable Y depends on variable X, then you may be tempted to estimate the model Yit = β0 + β1Xit + vit using OLS. However, your results would be flawed because OLS ignores the unique nature of the error term. The composite error term (vit) is vit = wi + εit. Although εit satisfies the classical linear regression model (CLRM) assumptions, the inclusion of wi in the composite error results in a CLRM assumption violation. (For a refresher on the CLRM assumptions, see Chapters 6 and 7.)