(FORTUNE Magazine) – If your company blew away its revenue projections and handily beat its earnings estimates last year, congratulations. But don't get too cocky. At the root of every figure on an income statement are the customers who spend their money with you, and many are fickle enough to find someone else to serve their needs at any moment. Consider, then, how nice it would be to track your relations with customers continually, to know for sure where you stand with your most important stakeholders.

The American Customer Satisfaction Index (ACSI), which appears with accompanying articles in the following pages, is a good place to start. Based on a massive survey of how U.S. consumers rate a wide range of products and services, it can help managers in a number of ways. If you're looking for broad industry trends or are trying to understand how customer satisfaction can help determine your company's financial results or stock price, it's definitely worth a careful look.

To come up with the ratings, which appear exclusively in FORTUNE, the National Quality Research Center canvasses more than 50,000 U.S. consumers each year. Statisticians at the University of Michigan take over from there, measuring the responses according to six different quality indexes. Then they score each of close to 200 companies, along with some major government services, on a 100-point scale.

This year's results are notable for the long-term trends they reveal and for some big one-year swings. While the four-year decline in the overall index has stopped, only one industry group has seen its scores improve over the life of this measurement: the cops. Seems as if all that community policing, with more officers walking the street and keeping an eye on the neighborhood, is making residents happy in addition to lowering crime rates. Surprising losers are the personal computer makers, whose aggregate scores have fallen 10% in four years. Sure, the machines are more powerful than ever, but what good are they if you can't get through to the help lines? Only broadcast news fell further over the same time period--a full 20%--which shouldn't surprise anyone at all.

Mercedes-Benz tops the list of individual companies. The IRS brings up the rear for the fourth straight year, though it also posted the biggest one-year percentage gain (8%). McDonald's is the IRS's closest neighbor at the bottom, putting the falling golden arches below the worst of the utilities and all the garbage men. Tied (with General Public Utilities) for the biggest one-year loser award is American Airlines, which plummeted 12.7%. Like most carriers, American jacked up fares for business travelers, then packed them into crowded planes--moves certain to annoy some of its best customers.

--Ronald B. Lieber