13. The non-transitory computer storage medium of claim 9 , wherein the instructions further cause the data processing apparatus to perform operations comprising: identifying one or more sets of matching web pages from the plurality of web pages, each set of matching web pages being a two or more web pages that each have a characteristic that matches a characteristic of another web page in the set of matching web pages; and identifying one or more sets of matching labels from the set of initial labels, each set of matching labels including two or more initial labels having at least a threshold measure of similarity.
13. The non-transitory computer storage medium of claim 9 , wherein the instructions further cause the data processing apparatus to perform operations comprising: identifying one or more sets of matching web pages from the plurality of web pages, each set of matching web pages being a two or more web pages that each have a characteristic that matches a characteristic of another web page in the set of matching web pages; and identifying one or more sets of matching labels from the set of initial labels, each set of matching labels including two or more initial labels having at least a threshold measure of similarity. 16. The non-transitory computer storage medium of claim 13 , wherein identifying one or more sets of matching labels comprises identifying at least one set of matching labels in which each label references a same topic.