Preventing and detecting certification errors in the Food Stamp Program (FSP) is a major policy concern. In 2005, the cost of overpayments was $1.29 billion, about 4.5 percent of the $28.6 billion in benefits issued. This report examines the State-level relationships between FSP certification error rates and certification expenditures, program policies, caseload characteristics, and economic conditions. The results show that, during the study period of 1989- 2005, a 10-percent increase in certification “effort”—about $35 per participating household—would reduce an index of certification errors by 2 percent (0.3 percentage points out of a mean of 15.1 percent). The effect of certification effort was significantly smaller between 1997 and 2002, when States were implementing welfare reform. Key simplification policies authorized by the 2002 Farm Bill were estimated to jointly reduce the error index by 4.4 percentage points.