As 15 million people in the United States head to airports this holiday season — slightly fewer than last year — some travelers will find welcome changes to security screening procedures.

The Transportation Security Administration has expanded its PreCheck trusted traveler program to 35 airports, allowing members who have been deemed low risk to keep shoes, jackets and belts on. Children 12 and under, and passengers 75 and older also receive expedited screening at any checkpoint; pilots, flight attendants, members of the military and people with top-secret security clearances qualify at some airports.

John S. Pistole, TSA administrator, said his agency’s priority this year has been to move toward a risk-based approach to screening, recognizing that the vast majority of travelers are not potential terrorists.

“When the agency was set up, it was focused almost exclusively on the security mission and not as much on the passenger experience,” Pistole said. “It became an adversarial relationship, so what we’re trying to do through all these initiatives is change that paradigm and make this a partnership.”

Even with these changes, the agency is under pressure to refine its strategy further in 2013. Its operations have been scrutinized by independent researchers, travel industry committees and government officials charged with oversight, and their ideas for reform are coalescing around a consistent theme.

“I use this acronym SEE,” said Stephen M. Lord, director of homeland security and justice issues for the Government Accountability Office, the investigative arm of Congress that has issued many lengthy reports about the TSA. “They need to make the process more selective, more effective and more efficient.”

More selective means “shrinking the haystack and really focusing on the dangerous people,” Lord said.

While PreCheck and other expedited screening options are a step in that direction, only 7 percent of passengers qualify for these programs, a number Pistole said the agency was working to expand.

One option being tested is to use dogs that sniff for explosives in tandem with behavior detection officers to divert more people to PreCheck lanes. That process was used at Indianapolis International Airport the day before Thanksgiving, allowing nearly a third of passengers to have expedited screening.

Regarding effectiveness, Lord said, the TSA needs to improve the technology it relies on — primarily expensive body scanners that may not detect explosives reliably.

Finally, becoming more efficient means addressing the time passengers spend waiting to get through security — a factor that the TSA does not measure consistently or make public, but one that is of growing concern to the travel industry as passenger volume has stagnated.

“You can’t focus exclusively on security,” Lord said. “You’ve got to be mindful of customer service.”