Economic historians vary in their calculations of the gross domestic product of the Roman economy during the Principate. In the sample years of 14, 100, and 150 AD, estimates of per capita GDP range from 166 to 380 "sestertii". The GDP per capita of Italy is estimated as 40 to 66 percent higher than in the rest of the Empire, due to a more advanced level of urbanization and trade, as well as tax transfers from the provinces and the concentration of elite income in the heartland.