Modelling word-pair relations in a category-based language model

A new technique for modelling word occurrence correlations within a word-category based language model is presented. Empirical observations indicate that the conditional probability of a word given its category, rather than maintaining the constant value normally assumed, exhibits an exponential decay towards a constant as a function of an appropriately defined measure of separation between the correlated words. Consequently a functional dependence of the probability upon this separation is postulated, and methods for determining both the related word pairs as well as the function parameters are developed. Experiments using the LOB, Switchboard and Wall Street Journal corpora indicate that this formulation captures the transient nature of the conditional probability effectively, and leads to reductions in perplexity of between 8 and 22%, where the largest improvements are delivered by correlations of words with themselves (self-triggers), and the reductions increase with the size of the training corpus.