A note on covariate measurement error in nonlinear mixed effects models

Little is known about the effects of measurement error in intra-individual covariates on inference for the nonlinear mixed effects model. We investigate this issue for a controlled variable measurement error model, and find that a major consequence may be substantial bias in estimates of parameters characterising intra-individual variation. Estimation of population parameters may also be affected. The dramatic effect of measurement error on estimation of intra-individual variance parameters has implications not only in the repeated measurement context, but-also in individual nonlinear regression models.