Research has demonstrated that cash incentives paid to respondents in sample surveys can increase the level of cooperation, reduce non-response bias, and lower data collection costs. However, recent research has shown that gains in response rate and reduced data collection costs associated with monetary incentives may vary across sub-groups in the population. Consequently, monetary incentives may result in inconsistent reductions in non-response error and systemic changes in sample composition. This paper describes an incentive experiment conducted as part of the 2001 National Household Survey on Drug Abuse (NHSDA), and evaluates the impact of monetary incentives on measures of cooperation for different population sub-groups. Findings indicate that the incentive had a positive impact on cooperation. Furthermore, the incentive neither introduced additional differences in cooperation propensities, nor did it eliminate the pre-existing differences in cooperation among population sub-groups.