Identify the epidemiologic study design that best fits the following study description. Researchers are interested in investigating the association between income and obesity in the United States. The data they have access to includes the median household income in each county collected in the 2000 U.S. census and the prevalence of obesity in each county during 2010, estimated by a national survey. They use these data to estimate the association between median household income in 2000 and prevalence of obesity in 2010