Document Type : Scientific Paper
http:// www.gpo.gov/fdsys/.../content-detail.html.
http://www.casebooks.eu/documents/tortLaw/heading5.1.2pdf
https://archive.org/stream/MedicalJurisprudenceAndToxicology/TXT/00000427txt
http://www.businessdictionary.com, under the words: skilled worker, semi-skilled and non-skilled.