Q: Your SIR model predicts peak at day 30, real peak day 35. Validate and check sensitivity to β. Validation: compare predictions vs data (residuals!)! Error: 5 days late? Check β: higher β → earlier peak. Sensitivity: ∂(peak day)/∂β: large → model fragile! Recalibrate β from data! Cross-validate: fit half, test half! (Models: trust but verify!)