The percentage of Polk County residents who have mailed back 2010 census forms is slightly above the national average and equal to the statewide average. According to the Census Bureau, 71 percent of Polk County residents have participated in the census by mail as of Monday. Both Henderson and Rutherford counties had return rates of 74 percent. The national average is 69 percent. Minnesota has the top rate in the country at 78 percent. South Carolina and Spartanburg County are at 68 percent. According to the Census Bureau, every percentage point increase in the national participation rate by mail saves about $85 million. Households have until mid-April to mail back their forms before census takers begin going door to door to residences that failed to respond.