Tests If Dropouts Are Missed at Random

Dropouts are a common problem in longitudinal investigations where individuals are measured repeatedly over time. This holds also in a study on rheumatoid arthritis where an inception cohort was followed up over three years. The question arose whether or not these individuals caused a selection bias. Two tests taken from the literature could be used to answer this question. But since they do not use all available information, a new asymptotic test is proposed. In a small simulation study it is shown that the new test is more powerful than the others.