A new study says Carver County is the state's healthiest county, while Cass County is the least healthy.   The County Health Rankings Report, prepared by the University of Wisconsin Population Health Institute, uses a standard formula to measure each county's health.   They use factors like people who die under the age of 75, health behaviors and social and economic factors.    Three area counties ranked in the top 20, with Mcleod at number 7, Yellow Medicine at number 8, Kandiyohi was ranked 12th and Redwood County was 18th.   One area county was in the bottom 20, Renville, which was ranked 74th out of the state's 87 counties.   (KWLM)