Systematic supervision procedures have been proposed to improve contingent valuation surveying, particularly in developing countries. Surprisingly, the CV literature does not say much about the potential effects of supervision even though there is evidence of interviewer effects and social desirability issues that can bias results. This paper investigates the effects of interview supervision on the valuation of public services, using split-sample treatments to include a test of scope of a nested good and to assess the effect of interview supervision on reported WTP. Results suggest that supervisors can be used to improve quality with no effect on WTP estimates.