SALT LAKE CITY — State Superintendent of Public Instruction Larry Shumway tried to explain Wednesday what caused a $25 million accounting error in the state education budget and how similar problems will be avoided in the future.

"We apologize," Shumway told the Education Interim Committee, speaking on behalf of the State Office of Education. "We're sorry it happened."

Shumway said the error was the result of a mathematical formula in an Excel spreadsheet referencing the wrong cell to calculate the state's weighted pupil units. That inaccurate figure was then used for a per-pupil funding estimate that resulted in public education being underfunded by the Utah Legislature.

Jonathan Ball, director of the Office of the Legislative Fiscal Analyst, said his office works cooperatively with the State Office of Education in determining the amount of funding necessary for student growth, but added that an oversight failure exists in the calculation of the weighted pupil unit. He suggested that a long term solution to avoid similar errors would be for his office to make its own weighted pupil estimate to compare to the office of education — a practice his office has already begun.

"Frankly, we've already done that in our office," Ball said. "Had we had this in place prior to the 2012 general session, we could have taken our number to the state office and they would have told where we were wrong."

Shumway told the committee that the office of education is altering its internal procedures and working with Ball to avoid errors in the future.

"Both our offices are working to ensure there are additional steps in place to have additional reviews," Shumway said.

But the question remains how the state will address the $25 million shortfall. Shumway said the practice of estimating student growth provides a margin of error, but added that the missing funds were too substantial for the State Office of Education to correct on its own.

"This error is of such magnitude that we are not comfortable we can handle it simply within the margins and the typical practices and the authority that we usually have," he said.

Rep. Francis Gibson, R-Mapleton, said the budgeting error was among the items listed on a letter signed by the chairs of the Senate and House education committees asking Gov. Gary Herbert to call a special session of the Legislature. Members of the House GOP caucus were told Wednesday the shortfall needs to be dealt with quickly.

"We feel as House leadership, we should address it sooner rather than later," House Majority Leader Brad Dee, R-Ogden, said, noting school districts need to finish their budgets by the end of June.

But Senate Republicans aren't in such a hurry, preferring to wait until later in the summer, after the current budget year ends.

Only the governor can call lawmakers into special session, and Herbert isn't ready to do that yet.

"The governor told the Senate president and the House speaker to work out their differences and bring us a consensus recommendation for consideration," Herbert spokeswoman Ally Isom said.

In response to Shumway's comments, members of the committee took an understanding tone, noting that errors in accounting are common. Rep. Ken Sumsion, R-Lehi, suggested to the committee that the Legislature shares some of the blame for the error due to its constant tinkering with the office of education's requirements in state statute.

"In fairness, part of it is we make it pretty complicated," he said.

But several lawmakers expressed concern that the budgeting of a $3 billion organization was being done on a program as simplistic as Excel. More sophisticated programs, they said, safeguard against human error and reduce the risk of miscalculation.

Shumway echoed their concerns, saying that the office of education has looked at alternatives to Excel, but emphasized that any professional accounting program would require a significant financial investment.

"Why are we relying on Excel?" Shumway said. "It's as close to being free as you can get."

Contributing: Lisa Riley Roche