GAO (2009) National Transportation System: Options and Analytical Tools to Strengthen DOT's Approach to Supporting Communities' Access to the System. From United States Government Accountability Office. July 2009.
http://www.gao.gov/new.items/d09753.pdf. Accessed March 11, 2010.

Since 1978, the Essential Air Service (EAS) program has subsidized air service to eligible communities that would otherwise not have scheduled service. The cost of this program has risen as the number of communities being served and subsidies to air carriers have increased. At the same time, the number of carriers providing EAS service has declined. Given continuing concerns over the EAS program's long-term prospects, GAO was asked to review the program. GAO reviewed (1) the characteristics and current status of the EAS program, (2) factors affecting the program's ability to provide air service, (3) options for revising the program, and (4) tools for assessing the program, the options for its revision, and the program's performance. GAO interviewed stakeholders and reviewed the results of an expert panel convened by GAO, Department of Transportation (DOT) data and program documentation, and potential methodologies for assessing federal programs.