1. A method for modifying a search query to retrieve documents relevant to a user's information need, the method comprising: compiling session data comprising a first query, a second query, a first set of relevant documents for the first query, a set of user interactions with a subset of the first set of relevant documents, and a set of query changes between the first query and the second query, wherein the set of user interactions comprises respective clicks on hyperlinked titles of the subset of the first set of relevant documents, wherein the set of user interactions further comprises respective amounts of time the user spent reviewing respective relevant documents of the subset of the first set of relevant documents; generating a feedback model estimating a relevance of search terms to the user's information need based on the set of query changes and the set of user interactions with the subset of the first set of relevant documents, wherein the set of query changes between the first query and the second query comprise at least one added search term that appears in the second query and does not appear in the first query, at least one removed search term that appears in the first query and does not appear in the second query, and at least one retained search term that appears in both the first query and the second query, wherein the feedback model is based on a similarity between the first query and the second query, wherein the similarity is based on a fraction having a numerator and a denominator; wherein the numerator comprises a sum of products comprising, for respective search terms appearing in both the first query and the second query, a first number of appearances of the respective search term appearing in the query having fewer instances of the respective search term, multiplied by a log of a quotient comprising a total number of documents in a corpus of documents as the numerator and a second number of documents containing the respective search term as the denominator; wherein the denominator comprises a sum of products comprising, for all respective search terms appearing in any of the first query and the second query, a second number of appearances of the respective search term appearing in the query having more instances of the respective search term, multiplied by a log of a quotient comprising a total number of documents in a corpus of documents as the numerator and a second number of documents containing the respective search term as the denominator; wherein generating the feedback model comprises: identifying a first portion of documents in the subset of the first set of relevant documents and associated with the at least one added search term and the at least one retained search term as having a higher relevance to the user's information need; and identifying a second portion of documents in the subset of the first set of relevant documents and associated with the at least one removed search term as having lower relevance to the user's information need; generating a relevance model estimating a relevance of search terms to the user's information need based on the feedback model and a session relevance model, wherein the session relevance model comprises a representation of the user's information need based on at least one query prior to the second query, wherein the relevance model comprises a weighting parameter applied to the session relevance model and the feedback model, wherein the weighting parameter is based on an information gain associated with the feedback model relative to the session relevance model; modifying the second query based on the relevance model, wherein modifying the second query further comprises adding at least one related search term to the second query based on the at least one added search term and the at least one retained search term, wherein modifying the second query further comprises deleting the at least one removed search term, wherein modifying the second query further comprises increasing a first weight associated with the at least one added search term and decreasing a second weight associated with the at least one removed search term; and outputting a second set of relevant documents to a user interface responsive to executing the modified second query, wherein the second set of relevant documents comprises documents associated with the modified second query, the first portion of documents, and wherein the second set of relevant documents does not contain the second portion of documents, wherein the outputting further comprises a respective hyperlinked title and summary of each document of the second set of relevant documents.