The article looks at how much money different regions in Norway made from 1900 to 2010. They gathered and organized data from 1965 onwards and used historical income info to go back to 1900. The best data on Norwegian incomes come from tax records. They used this data to figure out how much money each region made. The population info came from censuses done every 10 years starting in 1900. The researchers also looked at how income data was used for Great Britain during the same time period.