"Average Revenue per User" (ARPU) is, today, the accepted metric for measuring the success of mobile operators. However, in our most recent study, entitled "AMPU not ARPU: A BETTER METRIC FOR THE WIRELESS INDUSTRY," we argue that the ARPU metric is insufficient and that operators may limit their profits by focusing exclusively on it. "Average Margin per User" (AMPU) provides a basic - but generally unacknowledged - criterion for measuring the success of wireless operators. By focusing on AMPU, operators can generate profits sooner - and at higher rates - than otherwise would be the case. Over the past decade, the industry has expressed dismay at declining ARPU, notwithstanding actual increases. Part of this dismay stems from the assumption that declining ARPU implies a loss in profits. Few financial analysts have questioned whether low ARPU customers actually lose money for operators - or how operators could profit even if ARPU would continue to decline. Nor have they questioned whether high ARPU customers are consistently profitable. As a more thoughtful observer pointed out, "part of the problem [of measuring profitability] is [that] the Street doesn't believe carriers have the data to prove who's a profitable customer." With a renewed focus on profits, operators and financial analysts have shifted their focus to ARPU. This focus, however, is misplaced. The importance they attribute to ARPU rests on two widely articulated assumptions: (1) that margins for the lowest ARPU customers are inherently unprofitable and (2) that new data services will lift ARPU, and with that profitability. However, both of these assumptions are flawed. By adopting either, operators may extend losses rather than increase profits. There are two reasons for this. First, low revenue per user need not preclude a positive AMPU. In other words, low revenue users can still be profitable as long as ARPU exceeds average cost per user. For example, prepay customers have been widely assumed as unprofitable. Indeed, NTT DoCoMo lists "low rate of prepaid customers" as one of its advantages in the Japanese market. Prepaid customers may be low ARPU. However, they may generate higher revenue per minute than do post-pay customers. In addition, they require no handset subsidies, no billing and collection costs, and produce minimal bad debt. For these reasons, they can generate positive AMPU and with that, profits. Second, even though data services will raise revenues, the full costs of delivering data may exceed such revenues. If so, AMPU will be negative. As an illustration, numerous trade press articles have extolled the revenues - and supposed profits - inherent in UMTS services. Yet, to date, the costs of providing UMTS have proven a financial disaster. For such reasons, understanding AMPU becomes essential if operators are to increase profits.