For example, p(e 2 |f ) can be calculated straightforwardly using maximum likelihood estimation by counting how often the phrases e and f were aligned in the parallel corpus: There is nothing that limits us to estimating paraphrases probabilities from a single parallel corpus.
we can extend the definition of the paraphrase probability to include multiple corpora, as follows: where c is a parallel corpus from a set of parallel corpora C. multiple corpora may be used by summing over all paraphrase probabilities calculated from a single corpus (as in Equation 1) and normalized by the number of parallel corpora.