Private security screeners being used at some airports in the United States to screen passengers and their belongings in same cases perform better than the national averages for some performance measures and other cases below, according to a new report by the Government Accountability Office (GAO).

Private screeners at the majority of airports surveyed for the report passed their recertification tests to be able to continue to screen passengers and baggage, says the report, Screening Partnership Program: TSA Should Issue More Guidance to Airports and Monitor Private versus Federal Screener Performance (GAO-13-208). The report also says that airports with private screeners surpassed their respective airport category averages in about half the instances when objects were remotely projected on to X-ray screens of carry-on bags to test the proficiency of screeners.

On the other hand, GAO says the majority of airports with private screeners perform below their airport category averages when evaluated for factors that may affect security and a passenger’s overall traveling experience.

The Screening Partnership Program (SPP) is managed by the Transportation Security Administration (TSA) and allows airports to apply to opt out of having to use federal screeners. So far, 29 airports have applied to join the program and 16 are actively using private screeners. TSA is conducting competitions between private screening companies for awards to work at six more airports.

The report points out that in some instances performance differences at airports may not be entirely attributable to private or federal screeners due to differences in the types of passengers transiting those airports. For example, GAO says in airports near tourist locations may experience high volumes of first time or infrequent travelers, which are less likely to understand federal screening protocols and thereby require more bags to be screened and cause longer wait times.

In surveying three associations that represent airports, GAO also found that with two of the groups most of the airports they represent “are generally satisfied with the screening services provided by the TSA.”

While TSA collects data on performance measures, GAO says that they can’t be used to compare federal and private screener performance. For the report GAO didn’t present passenger wait time data at different airports because the TSA’s policy for collecting this data has changed and its collection has not been consistent across all airports.

GAO also says it was unable to review certain human resource measures such as absenteeism, attrition and injury rates because the agency doesn’t validate the data that private screening contractors send it nor does it require these contractors to use the same measures as TSA.

GAO says it is “incumbent on the TSA to be capable of determining if airports participating in the [SPP] program are performing at a level that is equal to or greater than the level of security that would be provided by federal screeners at the airports through regular monitoring and reporting.”

The TSA early last year had stopped accepting applications into the SPP program, saying there was no clear security benefit for using private screeners. Howerver, early this year Congress passed legislation that was subsequently signed by President Obama requiring the agency to restart the application process.