A metropolitan statistical area (MSA) is an area with at least one urbanized area and a population of at least 50,000 people. This includes the core jurisdiction plus adjacent counties that have a high degree of social and economic relations with the core county. A combined statistical area (CSA) is two or more MSAs or micropolitan statistical areas with employment interchange measures of at least 15. "Employment interchange measures" means "the sum of the percentage of workers living in the smaller entity who work in the larger entity and the percentage of employment in the smaller entity that is accounted for by workers who reside in the larger entity."