The effect of drop-out on the efficiency of longitudinal experiments

It is shown that drop-out often reduces the efficiency of longitudinal expe riments considerably. In the framework of linear mixed models, a general, c omputationally simple method is provided, for designing longitudinal studie s when drop-out is to be expected, such that there is little risk of large losses of efficiency due to the missing data. All the results are extensive ly illustrated using data from a randomized experiment with rats.