1. BLS Occupational Employment Statistics Query System, Bureau of Labor Statistics. Accessed various dates. https://data.bls.gov/oes/#/home.
2. “Certification and Licensing Status of the Employed by Occupation, 2019 Annual Averages.” Accessed April 25, 2020. https://www.bls.gov/cps/cpsaat53.pdf.
3. Characteristics of Persons in Scientific and Engineering Occupations: 1972. Technical Paper 33. Washington, DC: Department of Commerce, Bureau of the Census, Apr. 1974.
4. Economic Report of the President 2013. Washington, DC: Government Printing Office, Mar. 2013.
5. Employment and Earnings. Washington, DC: Bureau of Labor Statistics, various issues.