OLS assumptions

The infographic is a formula card stating the five Gauss-Markov conditions under which ordinary least squares is the Best Linear Unbiased Estimator (BLUE): linearity, random sampling, no perfect collinearity, zero conditional mean E(u|X) = 0, and constant variance Var(u|X) = σ². The most dangerous violation is E(u|X) ≠ 0, which biases the slope estimates; violating Var(u|X) = σ² — heteroskedasticity — distorts standard errors only, leaving the slopes unbiased.