12. A computer-implemented method for information retrieval, the computer-implemented method comprising: identifying a plurality of segments within a plurality of documents, wherein identifying segments includes analyzing the plurality of documents for features indicative of possible section headings, including at least one of: casing, spacing, punctuation, common words, or groups of words; accessing a concept hierarchy including a plurality of concepts of interest to the user, the concept hierarchy further including concept keywords associated with respective concepts; for each concept, determining statistical likelihoods that respective identified segments are associated with the concept, the statistical likelihoods each based on at least one of, for each combination of a particular concept and a particular segment: a density of particular concept keywords in the particular segment, wherein the density is based at least on a ratio of a quantity of particular concept keywords in the particular segment to a quantity of words in the particular segment; or a distribution of particular concept keywords within the particular segment, wherein the distribution is based on at least one of a longest span in the particular segment without any mention of particular concept keywords or a median gap between consecutive mentions of respective concept keywords in the particular segment; generating an index from the plurality of concepts and the statistical likelihoods that respective concepts are in each of the determined respective segments; and storing the index in a non-transitory computer storage.
20. The computer-implemented method of claim 12 , wherein the distribution of particular concept keywords within the particular segment is further based at least on a relationship G, wherein relationship G is defined substantially as: G = M 1 M 2 , where M 1 comprises the longest span in the particular segment without any mention of particular concept keywords, and M 2 comprises the median gap between consecutive mentions of respective concept keywords in the particular segment.