A voting system or electoral system consists of the set of rules which must be followed for a vote to be considered valid, and how votes are counted and aggregated to yield a final result. It is a method by which voters make a choice between candidates, often in an election or on a policy referendum. Common voting systems are majority rule, proportional representation or plurality voting with a number of variations and methods such as first-past-the-post or preferential voting. The study of formally defined voting systems is called social choice theory or voting theory, a subfield of political science, economics, or mathematics.