Some embodiments provide a method for identifying an event for a particular category. The method classifies several documents as relevant to several different categories. The method identifies a number of documents relevant to the particular category for a current time period and a background time period. Based on a comparison of the number of documents from the current time period relevant to the particular category and the number of documents from the background time period relevant to the particular category, the method identifies an event for the category for the current time period. Some embodiments calculated a score for the event, and normalize the score based on an average number of documents relevant to each of a set of related categories including the particular category.