Finite mixture model of conditional dependencies modes to cluster categorical data.

We propose a parsimonious extension of the classical latent class model to cluster categorical data by relaxing the class conditional independence as-sumption. Under this new mixture model, named Conditional Modes Model, variables are grouped into conditionally independent blocks. The correspond-ing block distribution is a parsimonious multinomial distribution where the few free parameters correspond to the most likely modality crossings, while the remaining probability mass is uniformly spread over the other modality crossings. Thus, the proposed model allows to bring out the intra-class de-pendency between variables and to summarize each class by a few character-istic modality crossings. The model selection is performed via a Metropolis-within-Gibbs sampler to overcome the computational intractability of the block structure search. As this approach involves the computation of the integrated complete-data likelihood, we propose a new method (exact for the continuous parameters and approximated for the discrete ones) which avoids the biases of the bic criterion pointed out by our experiments. Finally, the parameters are only estimated for the best model via an em algorithm. The characteristics of the new model are illustrated on simulated data and on two biological data sets. These results strengthen the idea that this simple model allows to reduce biases involved by the conditional independence assumption and gives meaningful parameters. Both applications were performed with the R package CoModes1 where the proposed model is implemented