16. The computer program product of claim 13 , further comprising displaying on the display device the influence of each keyword in each retrieved document.
13. A non-transitory computer readable storage medium having computer readable program code embodied therewith, the computer readable program code executable to cause a processor to perform a method for operating an information retrieval system, the method comprising: receiving, via a processor, a search query from a user, wherein the search query comprises a plurality of keywords; calculating, via the processor, relevance to a plurality of documents on the basis of the plurality of keywords and an influence set for each keyword, wherein the relevance is calculated a mathematical product of: a number of keywords in a search query in a document; a frequency of a keyword appearing in the document; a proportion of documents containing the keyword; a weight of the keyword; and a weight for a date indicative of when a search index was created; displaying, via the processor, on a display device documents in the order of relevance; displaying on the display device, via the processor, an influence set for each keyword; receiving, via the processor, changes to the displayed influence by the user; and recalculating the relevance, via the processor, on the basis of a change to the influence and displaying on the display device documents in the order of relevance.