Another challenge is how to achieve a balance between performance and rule table size. Note that given a source phrase, there would be plenty of "potential" candidate target phrases in weighted matrices<cite> (Liu et al., 2009</cite> ). If we retain all of them, these phrase pairs would produce even more hierarchical rules. For computational tractability, we need to design a measure to score the phrase pairs and wipe out the low-quality ones. We propose a new algorithm to calculate the relative frequencies of rules, and construct a measure that incorporates both frequency and lexical weight to score target phrases.