In general, cartel agreements are economically unstable in that there is an incentive for members to cheat by selling at below the cartel's agreed price or selling more than the cartel's production quotas. Many cartels that attempt to set product prices are 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. Once a cartel is broken, the incentives to form a new cartel return, and the cartel may be re-formed. Publicly known cartels that do not follow this business cycle include, by some accounts, OPEC.