The Office of the Comptroller of the Currency issued a warning Tuesday to national banks that use computerized credit scoring models.

"Credit scoring models can be used well or badly," Comptroller Eugene A. Ludwig said. "Because they are so powerful, their potential for good-or ill-is magnified. That means they must be developed, implemented, tested, and maintained with extreme care."

Banks use credit scoring models to evaluate potential borrowers and underwrite retail credit, including credit cards, residential mortgages, and home equity and small-business loans. While these tools can help bankers control risk and speed loan approvals, they pose potential risks and compliance pitfalls, according to a bulletin sent to national banks and OCC examiners.

The agency said banks should establish clear, written credit scoring policies. Credit scoring models should be applied to the products, customers, and loan sizes they were designed to judge, according to the OCC's bulletin.

Bank management should lay out how employees who use credit scoring models are trained and supervised, and detail under what circumstances the models' findings may be rejected.

The agency cautioned against overriding the models' predictions too often. By ignoring objective findings, loan officers may rely too much on subjective information and could inadvertently break fair-lending rules, Wayne Rushton, senior deputy comptroller for bank supervision policy, explained in an interview Tuesday.

"Overriding too often creates a risk that lending criteria will not be uniformly applied to all loan applicants," he said. "On the other hand, sometimes bankers are looking to stress growth of a particular loan area, and that's O.K. as long as they do overrides on a fair and equal basis."

Even when using credit scoring models, there is potential to discriminate against loan applicants, according to the Comptroller's Office.

For example, banks could be cited for fair-lending violations if, before data is entered into the model, assistance is provided to improve one applicant's qualifications and not others.

The comptroller's bulletin also recommended:

Reviewing credit scoring models, regardless of whether they are designed by the bank or purchased from outside vendors.

Comparing actual performance of loan portfolios with the models' predictions.

Ensuring that computer systems are powerful enough to handle and test the models.

Kevin M. Blakely, group executive vice president for risk management at KeyCorp, Cleveland, said that the comptroller's warning is warranted.

"With all the consumer losses that have cropped up, it's a good time to pause and think about the accuracy of your scoring models," Mr. Blakely said. "This bulletin will help focus bankers on important areas that they may just get too busy to think about."

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