1. A computer-implemented method for determining a proximity between a search query and an advertisement, comprising: preliminarily selecting, with one or more processors, an advertisement based on a set of search keywords associated with a search query, determining, with the one or more processors, a set of ad keywords associated with the advertisement; mapping, with the one or more processors, the ad keywords to at least one keyword cluster, where each keyword cluster is a group of terms related to a topic of the advertisement; computing a first conditional probability that the ad keywords occur in the at least one keyword cluster; computing a second conditional probability that the search keywords occur in the at least one keyword cluster; and determining, with the one or more processors, a topical proximity between the search query and the advertisement from a comparison of the first conditional probability and the second conditional probability.