The more loyalty becomes complex, the more one man says it should be simplified. Fred Reichheld, author and director emeritus of Bain & Company, has struck a nerve with his book The Ultimate Question and his concept of the Net Promoter Score (NPS). The ultimate question is: "Would you recommend this product or service to an associate, friend, or family member?" On a scale of one to 10, customers are asked how strongly they would recommend a company. The NPS is derived from indexing two numbers from that scale. The first is measuring promoters (those who score a company as a nine or 10).

The second is the detractors (those who score a company from zero to six). Subtracting the percentage of detractors from the percentage of promoters gives an NPS. After three books on loyalty and a whole career studying its complexities, Reichheld believes the NPS is the best indicator of customer loyalty.

"You can look at raw data and generate a lot of numbers about customer loyalty. Those numbers are all important but they're not good enough," Reichheld says. "The best measure of loyalty is investment. Will customers invest the time and effort to come back and buy your goods or services at full price? More important, will they invest the time and effort to refer you to friends and provide your company with the kind of feedback that will provide other measurable results, such as profit?"

Reichheld's theory is being tested at several companies. Research from Bain and Satmetrix shows that Net Promoter overachievers are consistent with the most trusted and successful companies in the world. Companies with the highest Net Promoter scores include USAA (82%), Homebanc (82%), Harley-Davidson (81%), Costco Wholesale (79%), Amazon (73%), eBay (71%), and SAS (66%).

Enterprise Rent-A-Car gets a 90 percent response rate to the one-question Net Promoter survey, which is asked by phone shortly after a customer's rental is complete. According to Reichheld, Enterprise CEO Andy Taylor says that a high NPS is the best predictor of referrals and repurchase. In fact, Taylor is so convinced of its importance that employees' promotion criteria are based in part on Net Promoter survey results. And branch rankings are published monthly to inspire peak performance. Reichheld frequently quotes Taylor's mantra: "The only way to grow a business is to get customers to come back for more and tell their friends."