Missing Data Mechanisms

Derives Rubin's MCAR/MAR/MNAR framework rather than just naming it: why complete-case analysis is exactly unbiased under MCAR, why it breaks under MAR but conditioning on the right covariates fixes it completely (a full derivation of the ignorability condition), and why MNAR admits no fix from the observed data alone. Covers the real remedies each mechanism licenses -- multiple imputation with Rubin's combining rules, maximum likelihood, and inverse probability weighting -- and ends with a simulation where covariate adjustment eliminates bias under MAR but only cuts it in half under MNAR.