Informative Drop-Out in Longitudinal Data Analysis – Discussion

A model is proposed for continuous longitudinal data with non-ignorable or informative drop-out (ID). The model combines a multivariate linear model for the underlying response with a logistic regression model for the drop-out process. The latter incorporates dependence of the probability of drop-out on unobserved, or missing, observations. Parameters in the model are estimated by using maximum likelihood (ML) and inferences drawn through conventional likelihood procedures. In particular, likelihood ratio tests can be used to assess the informativeness of the drop-out process through comparison of the full model with reduced models corresponding to random drop-out (RD) and completely random processes. A simulation study is used to assess the procedure in two settings: the comparison of time trends under a linear regression model with autocorrelated errors and the estimation of period means and treatment differences from a four-period four-treatment crossover trial. It is seen in both settings that, when data are generated under an ID process, the ML estimators from the ID model do not suffer from the bias that is present in the ordinary least squares and RD ML estimators. The approach is then applied to three examples. These derive from a milk protein trial involving three groups of cows, milk yield data from a study of mastitis in dairy cattle and data from a multicentre clinical trial on the study of depression. All three examples provide evidence of an underlying ID process, two with some strength. It is seen that the assumption of an ID rather than an RD process has practical implications for the interpretation of the data.