Funding Sources and Revenues While most progressive think tanks acquire their funding through many different sources, for conservative think tanks financing by corporations prevails. Among the think tanks receiving a considerable amount of corporate money are the Cato Institute (U.S. conservative/libertarian), the Brookings Institution (U.S. centrist), the Heritage Foundation (U.S. conservative), the American Enterprise Institute (U.S. conservative) and the Competitive Enterprise Institute (U.S. conservative). Also, whereas the combined revenue base of such conservative multi-issue policy institutions as the Heritage Foundation, the American Enterprise Institute, Free Congress Research and Education Foundation, the Cato Institute, and Citizens for a Sound Economy exceeded US$ 77 million in 1995, the roughly equivalent U.S. progressive Institute for Policy Studies, the Economic Policy Institute, Citizens for Tax Justice, and the Center for Budget and Policy Priorities had only US$ 9 million at their collective disposal in 1995.

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Expert system Expert systems are advanced computer programs that mimic the knowledge and reasoning capabilities of an expert in a particular discipline. Their creators strive to clone the expertise of one or several human specialists to develop a tool that can be used by the layman to solve difficult or ambiguous problems. Expert systems differ from conventional computer programs as they combine facts with rules that state relations between the facts to achieve a crude form of reasoning analogous to artificial intelligence. The three main elements of expert systems are: (1) an interface which allows interaction between the system and the user, (2) a database (also called the knowledge base) which consists of axioms and rules, and (3) the inference engine, a computer program that executes the inference-making process. The disadvantage of rule-based expert systems is that they cannot handle unanticipated events, as every condition that may be encountered must be described by a rule. They also remain limited to narrow problem domains such as troubleshooting malfunctioning equipment or medical image interpretation, but still have the advantage of being much lower in costs compared with paying an expert or a team of specialists.

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