1. A method of determining a feature related to an indication of a concept, the method comprising: selecting a set of positive cases using the indication of the concept, wherein the indication of the concept is a keyword; selecting a set of negative cases using the indication of the concept; training, by a computer system, a classifier for the concept using the set of positive cases in a manner blinded to the indication of the concept; applying the classifier to the set of negative cases; determining a feature related to the indication of the concept using results of the applying of the classifier, wherein the feature related to the indication of the concept is a related keyword to the indication of the concept; and generating a query including at least one of the keyword and the related keyword to perform a search.