9. A non-transitory computer-readable storage medium having executable computer program instructions embodied therein for determining category classifiers applicable to digital media items of a digital media repository, actions of the computer program instructions comprising: accessing a category-instance repository comprising relationships between categories and instances of categories, the category-instance repository derived from a corpus of documents comprising textual portions, the derivation comprising computing strengths for relationships between categories and instances based at least in part on frequencies of co-occurrence of the categories and instances over the corpus of documents; accessing a set of media item concept classifiers derived from the media items and associated with concepts derived from textual metadata of the digital media items in the digital media repository; computing consistency scores for a plurality of the categories based at least in part on scores obtained from media item concept classifiers associated with concepts corresponding to the instances of the plurality of categories; selectively removing the categories of the category-instance repository based at least in part whether the computed consistency scores indicate a threshold level of inconsistency; and determining, for each category of a plurality of the categories not removed, a category classifier based at least in part on the media item concept classifiers of concepts associated with the category, the determined category classifier when applied to a media item producing a score indicating whether the media item represents the category for which the category classifier was determined.
14. The non-transitory computer-readable storage medium of claim 9 , the actions of the computer program instructions further comprising: applying one of the determined category classifiers to one of the media items, thereby obtaining a score; and responsive to the obtained score indicating that the media item represents the category, adding a textual representation of the category to the textual metadata of the one of the media items.