BY BETTY LIN-FISHER, Knight Ridder Newspapers

When it comes to credit scores, it's the details that count. Credit scores are those elusive numbers that for years were secretive figures only seen by lenders deciding whether to approve a consumer's application. Within the last year, the three major credit bureaus - Equifax, Experian and TransUnion - have begun offering credit scores to consumers in addition to detailed credit reports that show your credit history.

Credit scores assign a number to a person based on the information in an individual's credit report using complex mathematical equations. The number, which summarizes the information in your credit report, is designed to tell credit lenders the likelihood that a consumer will repay a loan.

Jan Davis, executive vice president of consumer products for TransUnion, compared a credit score to a trip to the doctor: "Your credit score is kind of like your cholesterol number. You can't impact the number directly. You have to look at the underlying reasons," she said.

Scores have gained popularity since the early 1990s when mortgage lenders began using the figures to make the loan-approval process more objective. Scores are now used by virtually all lenders for loans for homes, autos and credit cards.

There are literally hundreds of different models designed by businesses to come up with a credit score, though the most widely used is what's known as the FICO score. The FICO score ranges from 300 to a high of 850 and is produced by Fair, Isaac and Company, which pioneered the technology for credit scores in the late 1950s.

Until 1989, the company was developing custom credit scoring models for individual lenders, many of which are still used. But now, at least three quarters of all mortgage decisions use the FICO score as do 23 of the largest 25 credit-card issuers and 40 of the 50 largest financial services institutions, said Craig Watts, consumer affairs manager for Fair, Isaac and Company.

Credit scores were originally designed for lenders, and the company has had to work hard to translate a lot of the "insider" language for lenders into user-friendly information for consumers, Watts said. Fair, Isaac has partnered with Equifax to offer the FICO score through the credit bureau. Competitors TransUnion and Experian offer their own versions of credit scores to the general public.

But part of the potential confusion among consumers can come up when they find out their credit score from one of the bureaus and assume the credit lender is looking at the same score. Lenders have access to the FICO score from the three bureaus, but consumers only have access to those FICO scores through Equifax.

So, I may apply for a loan with my Equifax credit score in hand, but my lender may have pulled a FICO score using the data in my Experian credit report.

Or the lender may have gotten my FICO score based on my TransUnion credit report, which may have slightly different information based on what is reported by my creditors (not all creditors report all of your history to all three bureaus, which is why it's important to check your credit reports from all three bureaus for inaccuracies).

Or another scenario may have my lender looking at a totally different score by another vendor. The bottom line is that you should always ask your lender what scoring model they are using, said Paul Richard, executive director of the Institute of Consumer Financial Education.

Though the scales for the scores from the three credit bureaus vary slightly, they all look at similar factors in credit reports when determining the score - for instance, maybe you have too much debt, too little credit history or a history of late payments.

While many consumers get caught up in the actual number that makes up their credit score, experts say what's more important are the factors that went into the score.

"Just like your cholesterol number, you can't really affect the number, you have to affect the behaviors," Davis said. All the different scoring models list the risk factors - or credit behaviors - that are negatively affecting your score. Fixing the negative factors could help raise your score in the future.

"They always sound negative, whether you have a great risk score and are very creditworthy," said Rod Griffin, spokesman for Experian.

A few of the scoring models also list the positive things raising your score, too.

Whether your actual score makes or breaks a loan might depend upon the lender. It might be more like your score opens or closes a door further to a loan.

Fair, Isaac's Watts said the guidelines of influential mortgage agencies Freddie Mac and Fannie Mae don't set rules, but their indicators set the standards for the mortgage lending industry. Those agencies, which buy mortgages from banks and resell them to investors, have indicated to lenders that any consumer with a FICO score above 620 is good while consumers below 620 should result in further inquiry from the lender, Watts said.

But the decision is still in the lenders' hands. For instance, one person may go to two lenders using the same credit score model on the exact same day and may get approved by one and rejected by the other based on the lending experience, Griffin said.

Stan Foraker, vice president of residential mortgage lending for Fifth Third Bank, Northeastern Ohio, said the credit score is only one indicator his bank looks at when deciding about a mortgage. "We don't stop at a credit score. We actually analyze the credit," said Foraker. "Someone with a moderately low (FICO) credit score, 620, could still have perfect repayment history, but they just have a lot of credit."

Credit scores are also an ever-changing number. Though they probably won't change dramatically, the score will vary according to what is in your credit report at a given time and what information about your payments - or lack of payments - to your creditors is shared with the bureau.

"Scores for the most part have no shelf life," said Watts, who added that lenders will typically look at your score a few times during the loan approval process to make sure nothing dramatic has changed in your file. But even with all the advice about how the reasons behind a credit score are the most important, it's still hard for consumers not to get bogged down by that number.

"We're so driven by numeric assessment of our performance in the United States, it's almost irresistible," Watts said. The people who get caught up in the credit score and a desire to get a perfect FICO score of 850 are the people who call most often, industry experts say.

But, Watts said once you get into the upper echelon of FICO scores - in the high 700s - lenders don't care how high your score is or isn't. You're still going to get a good lending rate. In actuality, the higher you get, the less attractive you are to lenders because they can't make money from you. "You get a high score by taking on little debt, managing it, not having very many credit accounts and not being susceptible to credit pitches from lenders," Watts said.

It's also difficult to try to outwit the FICO scoring method. For instance, Watts said opening or closing accounts to try to influence your score may or may not help or hurt your score. It really depends upon the mix of credit you already have, he said.

Credit bureau officials say they don't offer the same scoring model because "one shoe doesn't fit all." Yet Experian has altered their scale given to consumers to mirror FICO's scale to try to cut down on confusion.

TransUnion's scale is slightly wider, so it doesn't mirror FICO, but Davis told me I should have a pretty good idea of my potential FICO score by looking at the TransUnion score (which comes free with a report) and underlying factors going into my score as opposed to paying a little extra for a score from competitors.

Davis also readily acknowledges, however, that if you're going to try to get a mortgage tomorrow from a lender, "it's hard to beat the FICO score." So if the FICO score is the one that most lenders are using to make decisions on my credit worthiness, shouldn't I just go directly to the source and get a FICO score? If we're going back to the cholesterol scenario that Davis brought up, I see getting the FICO score as my first step and getting a score from a competitor as my second opinions.

Or better yet, Watts said his company is talking to Experian and TransUnion about also offering the FICO score to consumers so there won't be any confusion.