Thomas (1999), in a study of women in South Africa, finds a strong and statistically significant negative correlation between years of schooling and number of children born, even after controlling for potential confounding variables, as does Oliver (1999) in his study of women in Ghana. In China, for example, between 1970 and 2003, the share of manufacturing and mining in overall output increased from 28 to 60 per cent, while the share of agriculture dropped from 49 to 12 per cent, following a push in agricultural productivity and the development of diversified industries. By comparison, South Asia showed less dynamism, with its share of manufacturing and mining peaking in the 1990s to 22 per cent (compared with 14 per cent in 1970). Sub-Saharan Africa achieved little structural transformation, with agriculture continuing to be the mainstay of the economy.