This model being conditioned on several pieces of information, it can be affected by data sparsity problems.
the model is actually approximated as an interpolation of probabilities: where λ i , i = 1, ..., 4, are parameters of the model to be tuned, and c h is the cluster of head words for a given entity tag t. With such model, when not all pieces of information are available to estimate reliably the probability with more conditioning, the model can still provide a probability with terms conditioned with less information.