A subtle error has been found in the monthly statements issued to several hundred Washington area residents who have money market accounts with Standard Federal Savings and Loan, a Gaithersburg-based institution with 19 local branches and 100,000 deposit customers.

The error resulted in interest underpayments to "200 to 300 customers," averaging about $5 each, according to Greg Snell, senior vice president of bank operations. He said corrections have been made so that customers will not lose any interest on their accounts because of the error, which resulted from a miscalculation of the average daily balance in the accounts. In general, the average daily balance is computed by taking the balance in the account at the end of each day, adding those balances up and dividing the total by the number of days in the account period.

Snell wouldn't rule out the possibility of additional undiscovered errors on customer statements. "There may be a few phone calls we will still get and if we do, we will make corrections," he said.

Meantime, one customer who called for a correction for an error on his November statement said yesterday he is still waiting for written confirmation from Standard Federal that a correction has been made. The customer, who is a journalist, estimated that the miscalculation had cost him about $5 in interest for one month.

Snell said corrections had been made when individual consumers called about errors and when Standard Federal itself found the mistake. In some cases, the correction was made without the customer ever knowing there was an error, he said.

However, Standard Federal "has not done a major mass mailing," he said, "because the situation doesn't warrant that."

The mistakes occurred, Snell said, after Standard Federal switched to a new in-house computer system; previously, the company had used an outside computer service. But the new computer understated the average daily balance of some accounts between mid-October and mid-December, he said, and as a result, the amount of interest paid was less than it should have been.

According to Snell, the interest underpayment "was not a whole heck of a lot." He estimated that it amounted to about $5 per person or $1,000 to $2,000 altogether.

He said he didn't know of any case in which the Standard Federal computer had overstated the amount of the customer's average daily balance or the overpaid interest due the account.

Because of the nature of the error, customers with active accounts, in which there were many deposits and withdrawals, were less likely to notice the mistake because of the complicated calculations required to determine the average daily balance of such accounts. Those with little or no activity in their account should have had little trouble spotting the error, however.

The customer who found the mistake on his November statement, for instance, had no deposits or withdrawals during that period. Instead, he had an opening balance of $10,455.66. So he knew something was wrong when he saw the average daily balance listed as $9,780.88.

"I called them up and they said the computer had screwed up on all the money market accounts," the man said. "They said they were flooded with calls, but that they had fixed the computer and were fixing the accounts."

The man was told that his December statement would contain the corrected amount of interest. But when the statement arrived, there was no correction of the earlier error. "I called Standard Federal again and they said to disregard the statement I had received; they said I was supposed to receive a new corrected statement with an enclosure explaining everything."

The customer is still waiting.