The Share, Coincidence and Dominance Measures for Itemsets: Extended Report

We introduce the measures share, coincidence and dominance as alternatives to the standard itemset methodology measure of support. We also redefine the confidence measure in this context. An itemset is a group of items bought together in a transaction. The support of an itemset is the ratio of transactions in which an itemset appears to the total number of transactions. The share of an itemset specifies the ratio of the count of items purchased together to the total count of items purchased in all transactions. The coincidence of an itemset is the ratio of the count of items in that itemset to the total of those same items in the database. The dominance of an item in an itemset specifies the extent to which that item dominates the total of all items in the itemset. Measures based on share have the advantage of reflecting accurately how many units are being moved by a business, a capability that current itemset methodology does not provide. In addition, the share can be extended to give an accurate picture of the financial impact of an itemset on the business bottom line.