Bureau of Labor Statistics (BLS) data on certifications and licenses collected from the Current Population Survey (CPS). 

Certifications and licenses are credentials that demonstrate a level of skill or knowledge needed to perform a specific type of job.

Certifications are issued by a non-governmental body, but licenses are awarded by a government agency and convey a legal authority to work in an occupation. People may have more than one certification or license; people with a license may also have a certification.