emember the old Jimmy Stewart movie, "It's a Wonderful Life"? It's Christmastime in early 1940s small town America. Stewart plays a banker at a savings and loan who has loaned money to the poor to help them build decent homes. Now he faces ruin at the hands of a heartless Lionel Barrymore, who has found a misplaced bank deposit, called in the auditors and started a run on Stewart's bank. But the "little people" who Stewart put his faith in prove him right. They stream in with their dimes and quarters and dollars to bail him out.

Fast forward half a century. A computer has replaced Jimmy Stewart. Whether you get a loan to buy a home depends on a computer-generated credit score that compares certain things about you. Things like how much money you earn, how long you've been using credit and whether you've made payments on time determine your credit worthiness.

"The lender wants to know, 'If I lend money to 100 or 1,000 or 10,000 borrowers with these characteristics, will 90 percent or 95 percent or 99 percent repay?'" says Peter L. McCorkell, senior vice president at Fair Isaac, a company that develops these scoring systems for lenders.

The trick here is you can't find out your score. Fair Isaac and other firms say that if they reveal the scoring system - and the exact criteria to determine that score - their services wouldn't be needed anymore . What you can do, however, is get copies of your credit reports from one of the credit reporting agencies. Those reports won't tell you your score, but they do tell you what lenders know about you and your borrowing history.

Based on that information, you can also make an educated guess about how the scoring system works. The scores that companies like Fair Isaac compile are sent to the credit reporting agencies as composite numbers. In addition to your salary and other factors mentioned above, here are some of the things that scoring agencies consider:

If all of this sounds arbitrary or unfair, remember that scoring systems have allowed department stores and other lending agencies to offer those "on-the-spot" credit approvals. You know the routine. You fill out some basic information on a card and five minutes later (if the computer is working properly), you're either approved or disapproved for a loan.

McCorkell argues that credit scoring has helped fuel the economic boom of the 1990s because it allowed those who grant credit to grant much more of it - 20 percent to 30 percent more, by his estimates - than they otherwise would have.

"Today more credit at lower rates is available to a broader spectrum of American consumers than anyone could have imagined just a generation ago," he says.

Edward P. Howard, an attorney at the Center for Law in the Public Interest in Los Angeles, agrees that credit scoring has helped provide economic growth without inflation. But he worries about what can go wrong. There are a lot of elements stored in the computer.

"Even a fairly simple credit scoring system is likely to have 10,000 or 20,000 different possible combinations," McCorkell says. That's a lot of information to keep straight. What if it gets scrambled up?

OK, Howard knows it isn't realistic to think that Jimmy Stewart is going to loan any of us the money to buy a home today. But he's worried about your financial viability - your ability to borrow money, get a job, buy a home or rent an apartment. It all depends on a database that's supposed to be fail-safe. Howard's favorite movie is "Dr. Strangelove," the Cold War satire in which a mad U.S. general finds a loophole that allows him to launch a nuclear strike against the Soviet Union. So back in the real world, could this one credit database have a flaw that could destroy your hopes of buying a new home?

There are no mad generals involved in credit scoring. But both the score and the statistics that go into it are top secret. If everyone knew how the scoring was done, no one would pay his company to do it, McCorkell says. Further, if people understood the scores, they could cheat. "If a borrower knows he needs 10 more points to qualify for a particular loan and that closing two bank card accounts will raise his score by 12 points, he can go out and close two accounts today," McCorkell says.

As it is, though, if you get a bad score and you're turned down for credit, you can't find out what elements pulled it down. Some things are obvious. A high income earns more points than a low income. But minorities get lower scores, too. "Minorities and low-income borrowers present a slightly larger risk," McCorkell says. When he was asked how an applicant can improve his credit score, he replied that you "shouldn't focus on credit scores, but on the responsible use of credit."

But, of course, we will try to improve our credit scores, won't we? There are certain things we do know. Fewer credit cards are better than several cards. Paying on time is a must.

In The Ultimate Credit Handbook, Gerri Detweiler says, "The more you look like other people who pay their bills on time, the more likely it is the computer will approve your application." Some of the things that weigh heavily are stability - both at home and on the job - and a good payment history.

The scoring system looks at how close you are to the limits on your cards, what you spend money on and how much you ask for in cash advances. Pay attention. But don't despair.

"Even if you are head over heels in debt," Detweiler says, "you can rebuild your credit and improve your score."