The mission statement for the Organization of Petroleum Exporting Countries states, "the mission of the Organization of the Petroleum Exporting Countries (OPEC) is to coordinate and unify the petroleum policies of its Member Countries and ensure the stabilization of oil markets in order to secure an efficient, economic and regular supply of petroleum to consumers, a steady income to producers and a fair return on capital for those investing in the petroleum industry." Explain why the OPEC members often disagree on how much petroleum each country should produce.