1. An apparatus for information retrieval, comprising: a computer system including a computer processor being programmed to receive a query from a remotely located user system via a network, the query specifying one or more numerical data constraints, and contextual constraints; said computer system including a memory storing a computer-searchable electronic index, the index comprising a plurality of entries, each of the index entries representing an item of numerical data extracted from at least one of a plurality of electronic source documents including one or more natural language documents, wherein the item of numerical data comprises a numerical value, a prefix corresponding to a degree of significant digits, and a unit of measure; said computer system processor being further programmed to determine the relevancy of the query to each of one or more of the index entries based at least partly on a comparison between the numerical data constraint and the index entry's item of numerical data, at least partly on a comparison between the contextual constraint and contextual information extracted from the corresponding source document from which the index entry's item of numerical data was extracted, and at least partly on an evaluation of at least one positional relationship between the index entry's item of numerical data and the contextual information within a given source document, and generate a response to the query based on the relevancy.