Calibration plot: predicted versus observed in-hospital mortality for POSPOM (PreOperative Score to predict PostOperative Mortality) in the validation cohort (n = 2,789,932). Observed in-hospital mortality (bold line) with 95% CI for deciles of risk (triangles) and smooth. Nonparametric calibration line (dash line) was created with the LOESS algorithm (i.e., a locally weighted scatterplot smoothing).