The goal of attribution is to identify which touch, of the many possible, is most (or partially) responsible for a conversion, so ROI can be calculated. First touch, last touch, and multi-touch are common attribution models. For example, a sale might begin with an ad, lead to an email campaign, and end with a phone call from a salesperson. With the first-touch attribution, the ad would get the entire credit for the sale. With last-touch, the phone call gets all the credit. With multi-touch, the ad, the email and the phone call each get partial credit.