Many sources use numerical industry classification systems to organize information on industries and companies. The most commonly used ones in the U.S. are the Standard Industrial Classification (SIC) scheme and the North American Industry Classification System (NAICS). Other sources use words to define industries: air transport, apparel, banking, etc. Some industries are narrowly defined, and their scope may be understood only through the study of published literature and information from the involved companies and trade associations.
How to determine the NAICSs and SICs for a particular company? Company entries in sources such as ReferenceUSA and LexisNexis Academic often include NAICS and SIC codes in which the company operates.
Industry classifications and definitions can vary from source to source and change over time, making comparisons problematic. Even rather specific industry codes may be too general, and companies are often involved in numerous industries. It may be possible to derive reasonable estimates from general figures.