As the lower wage level in developing economies is reflected in lower prices of goods that are not traded internationally, one unit of the local currency will have greater purchasing power domestically than worldwide. For that reason, the World Bank has used purchasing power parity (PPP), which is calculated by converting a local currency into a reference currency (US$) in terms of purchasing power equivalent. Following this logic, in 1991, an international poverty line was estimated equivalent to one PPP dollar per day at 1985 prices.