Link: http://www.cbo.gov/doc.cfm?index=12696

From the Congressional Budget Office: "Employees of the federal government and the private sector differ in ways that can affect compensation. Federal workers tend to be older, more educated, and more concentrated in professional occupations than private-sector workers.

CBO's study compares federal civilian employees and private-sector employees with certain similar observable characteristics [...]. Even among workers with similar observable characteristics, however, employees of the federal government and the private sector may differ in other attributes, such as motivation or effort, that are not easy to measure but that can matter a great deal for individuals' compensation. This analysis focuses on wages, benefits, and total compensation between 2005 and 2010."