In the RFE estimate, every rule NT P → α / β receives a probability in proportion with the times that α / β was covered by the NT label.
estimating the parameters under Maximum-Likelihood Estimation (MLE) for the latent translation structure model p(σ) is bound to overfit towards memorising whole sentence-pairs as discussed in (Mylonakis and Sima'an, 2010), with the resulting grammar estimate not being able to generalise past the training data.