This paper addresses the question of whether administrative data sources, such as performance monitoring data, can be used for program evaluation purposes. It argues that under certain circumstances, such data can be used. In particular, program performance data that are routinely gathered and monitored by administrators of many workforce development programs meet these circumstances. The paper goes on to demonstrate the point by using administrative data from the state of Washington to examine the net impact on earnings and employment of services provided to adults under the Workforce Investment Act (WIA). Because of a lack of consensus about appropriate net impact estimators, the strategy of this paper is to examine the sensitivity of the results to various estimation techniques. The paper describes the various estimation techniques, and it summarizes the net impact estimates that are generated for the State of Washington. For the most part, the results are fairly stable across the techniques, which the paper argues adds a degree of confidence in them. The final section of the paper offers guidance to policymakers and program administrators who may not be familiar with the technical details of various analytical approaches about how empirical results that may appear to be complex or unstable can be used for program improvement.