We obtained data from 1,116 prepaid customers who participated once or twice in the CRP during the treatment period (January-December 2007). We ensured that these customers participated in the CRP only during the treatment period and not before or after. All participants successfully took part in the CRP such that the firm gained a new customer from their recommendations. The service provider's CRP required these new customers to indicate who had referred them to the service before it rewarded the recommender. Therefore, in all cases, both new and existing customers were aware of the referral reward. In contrast, the service provider could not observe unsuccessful recommendations