A new transaction set is compared to a plurality of transaction sets represented in a Compressed Frequent Item Set (CFIS), wherein the CFIS maintains a count for each transaction set represented in the CFIS. When the new transaction set matches a transaction set represented in the CFIS, the count for the matching transaction set in the CFIS is incremented. When the new transaction set does not match any transaction sets represented in the CFIS, the new transaction set is added to the CFIS. If there are intersections between two or more events of the new transaction set and the events of the plurality of transaction sets represented in the CFIS, the count for the transaction sets in the CFIS that intersect with two or more of the events of the new transaction set is incremented.