In general, cartel agreements are difficult to negotiate because potential members typically have different ideal collusive prices. Once formed, cartels tend to be economically unstable, primarily because there is a profit incentive for members to cheat by selling at below the agreed price or selling more than the production quotas set by the cartel (see also game theory). Cheating on prices is difficult for cartel members to observe, so more successful cartels often agree to fix their market quotas, share verifiable information about those shares, and agree in advance on some mechanism to punish members that exceed their quotas. This has caused many cartels that attempt to set product prices to be unsuccessful in the long term. Empirical studies of 20th century cartels have determined that the mean duration of discovered cartels is from 5 to 8 years. However, once a cartel is broken, the incentives to form the cartel return and the cartel may be re-formed. Publicly-known cartels that do not follow this cycle include the Organization of the Petroleum Exporting Countries (OPEC).