Usually the IBM Model 1, developed in the statistical machine translation field (Brown et al., 1993), is used to construct translation models for retrieval purposes in practice.
given a number of parallel strings, the IBM Model 1 learns the translation probability from a source word s to a target word t as: where λ s is a normalization factor to make the sum of translation probabilities for the word s equal to 1, N is the number of parallel string pairs, and J i is the ith parallel string pair.