Usability Statements of Work
http://www.usability.gov/pubs/010106news.html

Martin, Beth A.
Usability.gov
2006

Abstract:

Repeated (iterative) usability testing finds problems sooner, and usability engineering helps agencies save money. With those ideas in mind, it makes sense that agencies would want to submit their sites and other products to usability testing; doing so is an effective use of taxpayer money, especially with regard to performance measures.