1. A method for automatically determining whether pairs of words are synonym pairs, comprising: determining co-occurrence frequencies for pairs of words in documents; determining closeness scores for pairs of words in the documents, wherein a closeness score indicates whether words in a pair of words are located so close to each other in the documents that the words are likely to occur in the same sentence or phrase; generating correlations between (i) words in a title or an anchor of each document in the documents and (ii) words in the document; and determining whether each of the pairs of words is a synonym pair based on the determined co-occurrence frequencies, the determined closeness scores, and the correlations, wherein a high closeness score is a negative indicator that a pair of words is a synonym pair, a high co-occurrence frequency is a positive indicator that the pair of words is a synonym pair, and a high correlation is a positive indicator that the pair of words is a synonym pair; wherein the determining co-occurrence frequencies, determining closeness scores, generating correlations, and determining whether each of the pairs of words is a synonym pair is performed by a computer system.