1. A computer-implemented method for representing topical similarity among a plurality of text strings in a text string set, comprising: expanding the text string set using search results generated in response to at least one search query including the plurality of text strings thereby resulting in an expanded text string set; identifying frequent itemsets in the expanded text string set; developing a vocabulary for the text string set including selected ones of the frequent itemsets; calculating a similarity measure for each pair of the selected frequent itemsets in the vocabulary; and generating a topical coherence measure for the text string set with reference to the similarity measures.