Structure assignment to sentences: The use of probabilistic context-free grammars (PCFGs) was briefly discussed in  section 2. Supervised learning of PCFGs can be implemented much like supervised learning of HMMs for POS tagging. The required conditional probabilities of phrase expansion are easily estimated if a large corpus annotated with phrase bracketings (a treebank) is available (though estimates of POS → word expansion probabilities are best supplemented with additional data). Once learned, a PCFG can be used to assign probabilistically weighted phrase structures to sentences using the chart parsing method mentioned in section 2—again a dynamic programming method. 