Residual variability in population PK
Residual variability describes the difference between a concentration measurement and its individual model prediction.
Residual variability describes the difference between a concentration measurement and its individual model prediction (FDA. Population Pharmacokinetics. 2022.).
Under additive error, the standard deviation is constant on the concentration scale. With proportional error, it increases with predicted concentration. An individual with consistently low clearance can have elevated concentrations across visits even when residual error is small. Simulating measured concentrations generally requires residual error, whereas simulating latent exposure such as individual AUC generally does not.