See Rebecca Freeman, “Labour productivity indicators: comparison of two OECD databases - productivity differentials and the Balassa-Samuelson effect", OECD Statistics Directorate, July 2008. The ratio must be equal to one under the assumption of a competitive labour market, which implies that workers are paid the value of their marginal product and that firms hire up to the point where the marginal value product of labour equals the wage. This measure has some biases coming from various sources and data noise, yet even after considering sector differences in, for instance hours worked and the skill level of workers as well as alternative measures of sector output constructed from household survey data, a puzzlingly large gap remains. See Douglas Gollin, David Lagakos and Michael E. Waugh, “The agricultural productivity gap”, National Bureau of Economic Research Working Paper No.