1. A method for pruning a library of bi-phrases, comprising: partitioning a bi-phrase library into a set of sub-libraries; for each of at least a plurality of the sub-libraries and for each of a plurality of association score thresholds, with a computer processor, computing values of noise as a function of expected counts and observed counts, where observed counts corresponds to a number of bi-phrases in the sub-library having an association score above the association score threshold, and expected counts corresponds to a number of bi-phrases in the sub-library having an association score above the association score threshold under an independence hypothesis; and pruning the plurality of sub-libraries based on a common noise threshold, including, for each sub-library, determining an association score pruning threshold corresponding to the common noise threshold, and pruning bi-phrases from the sub-library having an association score which exceeds the association score pruning threshold.