For a CS of words, if we set the j-th word as an anchor word, the probability that all other words align to that word is: The alignment score of the whole CS is a weighted sum of the logarithm of the above alignment probabilities, i.e., and the global alignment score is computed as: Entropy model: In general, it is preferable to align the same word from different hypotheses into a common CS.
we use entropy to model the purity of a CS.