A think tank (or policy institute, research institute, etc.) is an organization that performs research and advocacy concerning topics such as social policy, political strategy, economics, military, technology, and culture. Most policy institutes are non-profit organizations, which some countries such as the United States and Canada provide with tax exempt status. Other think tanks are funded by governments, advocacy groups, or businesses, or derive revenue from consulting or research work related to their projects. The following article lists global policy institutes according to continental categories, and then sub-categories by country within those areas. These listings are not comprehensive, given that more than 4,500 policy institutes exist world wide.