Education is primarily a non-market service with the majority of pupils and students in participating countries receiving their education free or at prices that are not economically significant from non-market producers. Without economically-significant prices to value output, the expenditure on education provided by non-market producers cannot be derived as it is for market producers by summing their sales. To get round the problem, national accountants have adopted the convention of estimating expenditures on non-market services by summing their costs of production. Previously, to preserve consistency with the prices underlying the expenditure estimates, Eurostat and the OECD calculated PPPs for education and other non-market services with input prices: the input-price approach.