This paper investigates industry classification systems. During the last 50 years there has been a considerable discussion of problems regarding the classification of economic data by industries. This paper also examines the structure and content of industrial classification schemes and how they affect financial research.
Link
http://econpapers.repec.org/scripts/redir.pl?u=http%3A%2F%2Fsfb649.wiwi.hu-berlin.de%2Fpapers%2Fpdf%2FSFB649DP2005-062.pdf;h=repec:hum:wpaper:sfb649dp2005-062