Which level of schooling has the greatest economic impact on output?

An assessment is made of whether primary, secondary or tertiary education has the greatest impact on national income. An aggregate production function is estimated for a cross-section of 77 countries which disaggregates the labour force on the basis of the highest level of schooling attained. The results imply that for both high-income and less developed countries the marginal product of labour increases with each successive level of schooling. Tertiary education has the greatest economic impact.