A TEST OF THE MISSING DATA MECHANISM FOR REPEATED-MEASURES DATA

The occurrence of missing data is an often unavoidable consequence of repeated measures studies. Fortunately, multivariate general linear models such as growth curve models and linear mixed models with random effects have been well developed to analyze incomplete normally-distributed repeated measures data. Most statistical methods have assumed that the missing data occur at random. This assumption may include two types of missing data mechanism: missing completely at random (MCAR) and missing at random (MAR) in the sense of Rubin (1976). In this paper. we develop a test procedure for distinguishing these two types of missing data mechanism for incomplete normally-distributed repeated measures data. The proposed test is similar in spirit to the test of Park and Davis (1992). We derive the test for incomplete normally-distributed repeated measures data using linear mixed models. while Park and Davis (1992) derived the test for incomplete repeated categorical data in the framework of Grizzle Starmer and Koch (1969). The proposed procedure can be applied easily to any other multivariate general linear model which allow for missing data. The test is illustrated using the hip-replacement patient data from Crowder and Hand (1990).