Kotler, P., Armstrong, G., Brown, L., and Adam, S. (2006) Marketing, 7th Ed. Pearson Education Australia/Prentice Hall.
"Industry Definition & Meaning". http://dictionary.reference.com/browse/industry
International Labour Office (1999). Safety and health in agriculture. International Labour Organization. pp. 77–. ISBN 978-92-2-111517-5. Retrieved 13 September 2010. 978-92-2-111517-5
"Forestry." SAF Dictionary of Forestry. The Society of American Foresters, 1998. Helms, John A. <"SAFnet Dictionary | Definition for [forestry]". Archived from the original on 2013-10-19. Retrieved 2014-03-15.> https://web.archive.org/web/20131019122343/http://dictionaryofforestry.org/dict/term/forestry
Tor Selstad (1990). "The rise of the quaternary sector. The regional dimension of knowledge-based services in Norway, 1970-1985". informaworld. ... knowledge-based services ...
Peter Busch (1967). "Tacit Knowledge in Organizational Learning". Tacit Knowledge in Organizational Learning. ISBN 9781599045030. Retrieved 2010-06-17. see page .. The quaternary sector of industry is the sector of industry that involves the intellectual services. That is research, development, and information. 9781599045030