1. A computerized method for calculating the effectiveness of a promotion, the method comprising: receiving at a host computer system transaction data from a plurality of point of sale terminals that are associated with a plurality of merchants, wherein the transaction data comprises a merchant identifier for each of the merchants and a transaction amount for each transaction involving the merchants, and wherein one of the merchants comprises a promotion merchant that is offering a promotion involving transactions made with the merchant; aggregating a portion of the transaction data into control merchant aggregated data involving control merchants, wherein the control merchant aggregated data comprises transaction data obtained other than from the promotion merchant; calculating by the host computer system a characteristic of the purchases, both for transactions involving the promotion merchant and for the control merchants; providing an output showing a comparison of the calculation.