1. A method of disambiguating an ambiguously identified product category, the method comprising: receiving from a client machine, a search query comprising the ambiguously identified product category; processing the search query to identify item listings, via a database, that satisfy the search query; periodically deriving, for each item listing satisfying the search query, a dominant product category, the dominant product category being a numerical score based on a number of user-events of different types associated with an item listing, the score being used to predict the likelihood that the item listing will result in a transaction if the item listing is presented in a search results page, the dominant product category derived using a half-life formula such that a value for any particular event contributing to the dominant product category is reduced over time, as determined by the half-life formula, wherein the half-life formula has a configurable parameter representing a number of days until the value of an event contributing to the dominant product category is reduced by half, the configurable parameter being configurable on a per product category basis; providing via a network interface to the client machine, a first message comprising a plurality of images of the item listings hierarchically related by selectable product category aspects, the first message to be displayed at the client machine, each of the plurality of images relating to the same type of item, the plurality of images ranked by their dominant product categories; detecting via the network interface a selection at the client machine of more than one of the plurality of images that unambiguously identify the ambiguously identified product category; responsive to the detecting of the selection, providing via the network interface to the client machine, a second message comprising one or more selectable images of additional aspects of the ambiguously identified product category, the second message to be displayed at the client machine, the one or more selectable images of additional aspects unambiguously identifying the dominant product category as the ambiguously identified product category; and responsive to detecting the selection of at least one image of an aspect of more than one of the plurality of images that unambiguously identifies the product category or one or more selectable images of additional aspects of the product category, searching for listings of items of the dominant product category having the aspects represented by the at least one image of an aspect.