1. An information retrieval system having a storage device, a display device and a processor, wherein the processor is configured to: receive from a user a search query including a plurality of keyword; calculate a relevance to a plurality of documents on the basis of the plurality of keywords and an influence set for each keyword (t), wherein the relevance is calculated based on: a coefficient (coord(q,d)) determined by a number of keywords in a search query (q) in a document (d); a coefficient tf(t,d) determined by a frequency of a keyword (t) appearing in the document (d); a coefficient idf(t) determined by a reciprocal of a proportion of documents containing the keyword t; a weight (boost(t)) of the keyword t; a coefficient indicative of weight (norm(t,d)) indicative of when a search index was created; a coefficient (date(d)) determined by a date of the document d; and a weight for the date (dateBoost); and wherein the relevance (score) is calculated by: Score( q,d )=coord( q,d )×queryNorm( q )×(Σ tf ( t,d )×( idf ( t ))2×boost( t )×norm( t,d )+(date ( d ))×(dateBoost)); display on the display device documents in the order of relevance; display on the display device the influence set for each keyword; receive changes to the displayed influence set from the user; and recalculate the relevance on the basis of a change to the influence set and display on the display device documents in the order of relevance.