We propose a methodology for finding the empirical distribution of the Rand's measure of association when the two partitions only differ by chance. For that purpose we simulate data coming from a latent profile model and we partition them according to 2 groups of variables. We also study two other indices: the first is based on an adaptation of Mac Nemar' test, the second being Jaccard' index. Surprisingly, the distributions of the 3 indices are bimodal.