Counties are considered persistently poor if 20 percent or more of the population lived at or below the federal poverty line during four consecutive U.S. Census measurements dating back to 1980.
 
Counties are classified as metropolitan vs. non-metropolitan and persistently poor vs not persistently poor using the USDA Economic Research Service 2015 County Typology Codes, https://www.ers.usda.gov/data-products/county-typology-codes/

Population data is from the U.S. Department of Commerce, Bureau of the Census, PL-94, decennial census files, 2010 and 2020