Some cost assumptions made by the Transportation Security Administration comparing private-sector and federal screeners at airports weren’t sound in January 2009--and the problem still hasn’t been entirely resolved, the General Accountability Office told Congress.

Some airports use screeners who work for the federal government, while other airports entered into a program that allowed them to use private employees from approved companies.

But the fledgling security administration’s numbers, on which many airports would decide which course to take, overstated the financial benefits of using its own workers rather than private ones under what’s called the Screening Partnership Program (SPP).

“TSA had underestimated costs to the government for screeners at non-SPP airports because the agency did not include all of the costs associated with passenger and baggage screening services at these airports, such as workers’ compensation and general liability insurance, and certain retirement benefits,” the GAO’s Stephen Lord wrote to Rep. John Mica, chair of the House transportation committee Friday.

“Further, TSA did not reflect the revenue received by the government from corporate income taxes paid by SPP contractors. The omission of these factors reduced the reliability of TSA’s 2009 cost estimate by increasing the costs for private-contractor screeners relative to federal screeners.”

The TSA since adjusted some of its methodology, but “needs to do more” to address other concerns, and failed to adhere to standards for statistical analysis, the letter said.

In 2007, TSA estimated private screeners would cost airports “about 17 percent more to operate than airports using federal screeners. In its January 4, 2011, update, TSA estimated that SPP airports would cost 3 percent more to operate in 2011 than airports using federal screeners.”