The Government Accountability Office has released Tax Administration: IRS Can Improve Its Productivity Measures by Using Alternative Methods (GAO-05-671) (28 pages).  From the 1-page highlights:

GAO recommends that the Commissioner of Internal Revenue put in place a plan for introducing wider use of alternative methods of measuring productivity, such as those illustrated in this report, taking account of the costs of implementing the new methods. The Commissioner of Internal Revenue agreed with our recommendation and assigned responsibility for considering alternative methods of measuring productivity.