Analyze model diagnostic plots (e.g., residual, marginal model, QQ, and added variable plots) to assess quality of fit.

Free CAS MAS-I (Modern Actuarial Statistics I) lesson in Extended Linear Models. 15 min read, ~2,181 words.

Residuals vs fitted flags non-linearity (curved smoother) and heteroskedasticity (funnel); flat band near zero is the target. QQ plot of standardized residuals flags non-normal errors: S-shape is heavy tails, banana is skew, off-line endpoints are outliers. Scale-location plot isolates non-constant variance; flat red line means is stable across. Added variable...

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