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TESS VIGELAND: There are lots of ways to measure the importance of U.S. companies. Largest market cap? Exxon-Mobil. Most expensive share of stock? Berkshire Hathaway. The biggest influence on your financial life? A Minnesota-based company called the Fair Isaac Corporation. Over the last 30 years or so Fair Isaac's proprietary credit score, known as FICO, has arguably become the most important number in our lives. Lenders use it to determine whether to loan us money, and at what interest rate. Many employers use credit reports in screening job applicants, but those reports do not include the applicants' credit scores. So do insurance companies. Marketplace's Janet Babin has the story of one consumer and his credit score
that makes you wonder, just how fair is Fair Isaac?

Janet Babin: I'm about to introduce you to Pete Deibel. He's 35, and when it comes to managing his credit, he's just about perfect. He's rarely missed a credit card payment since college. When he and his wife bought a home in Winter Park, Fla., they qualified for the lowest mortgage rate. They timed the market just right and:

Pete Diebel: Immediately our property values doubled.

A few years later they refinanced. Once again the Deibel's high credit score earned them the lowest interest rate. They tapped their home equity line to pay off their credit card balances, student loans and new baby expenses. Like I said, Pete was a nearly perfect card-carrying credit risk.

Then the financial crisis hit, banks were forced to clean up their balance sheets. To reduce their own risks, they tightened lending standards for us. Ira Rheingold is executive director of the National Association of Consumer Advocates.

Ira Rheingold: Banks and credit card companies are trying to deal with all the bad loans that they've made and are responsible for, and are punishing consumers in the process.

Here's how Pete got punished. Without warning, the credit card companies slashed his credit limits. This even though he kept up his payments as he always had. With lower limits, his card balances instantly shifted from making up 20 percent of his available credit to 80 percent. Suddenly, Diebel looks like a guy who's living off his plastic. His FICO score dropped 10-points. That may sound miniscule on a scale of 300 to 850, but it could translate into thousands more in interest payments. Diebel didn't find out about the drop until he tried to refinance his home again. Here's what his mortgage broker told him:

Pete Diebel: Your score is a little lower than you thought it was, and I said, OK. And they also found a couple of things that were collections, that literally, we'd never seem them before.

Both charges were for medical procedures Diebel says he never even had. Complaints like this are on the rise, creating a backlog. Again, here's advocate Ira Rheingold.

Rheingold: Trying to get a credit report fixed is incredibly difficult. Oftentimes, unless you have an attorney helping you, you really can't get bad marks on your credit report removed.

To fix your score, you'll need to deal with at least three leading credit bureaus -- TransUnion, Equifax and Experian. Each company cooks up its own black box of methods to come up with your score. But Burt Flickinger with the Strategic Resource Group says the formula goes something like this:

Burt Flickinger: 35 percent is punctuality, about 30 percent is the amount of debt, 15 percent's the length of credit history, 10 percent's the type of credit used and the last 10 percent is recent credit and the amount obtained.

Flickinger says each credit bureau has its own version of your data, and a lot of times that data is wrong. He knows that first hand. A few years ago, Flickinger bought a condo. Bills for the former tenant's phone service kept coming to him.

Flickinger: My credit score would have been decimated, so I had to made the decision to pay the bills that weren't mine, and then request a refund.

While there are no hard numbers on credit bureau errors, advocates say they happen all the time. Steve Wagner with Experian says credit bureaus have to compile bits of information from more than 8,000 creditors. If a consumer complains about in incorrect charge:

Steve Wagner: We turn around and go back to the data provider and say, is this correct? And if the data provider comes back and says it's correct, then, we have to take that as being correct on the file.

Wagner says consumers should check their credit reports every three months to look for inaccuracies. But here's the rub: federal law only requires the companies to give consumers one free report each year. And that report doesn't even have to include your credit score, the FICO number. If you want to see your score more than once a year, you'll have to pay for it.

Credit reports were created to simplify the lending process. But Rheingold -- the consumer advocate -- thinks the pendulum has swung too far.

Rheingold: Maybe we should start looking at whether people have enough income to afford this credit, and take a look at what their debts are currently.

As for the guy who lost his gold star credit, Pete Deibel down in Winter Park? He's still looking for that new mortgage.

Deibel: We are going to fix these issues on our credit just as a matter of course. To be honest with you, I'm shopping rates.

Like a lot of us, Deibel may find the lowest possible interest rate just out of reach, or available only for a hefty fee.

I'm Janet Babin for Marketplace Money.