Kigali is the economic and financial hub of Rwanda, serving as the country's main port of entry and largest business centre. The NISR does not maintain detailed economic data for subnational entities in Rwanda, but economists have used various measures to estimate the city's output. A 2015 working paper by the World Bank Policy Research unit used the amount of light visible at night in different regions as a proxy for relative gross domestic product (GDP), and found that the three districts of Kigali represented 42 per cent of Rwanda's total night-light output. When translated, this gives a total city GDP of approximately US$1.8 billion or $1,619 per capita, compared with a national average of $436 per capita. Another 2015 World Bank study measured the total turnover of registered companies in the country, as reported to the Rwanda Revenue Authority, and found that 92 per cent of these were from the city of Kigali. However, the authors noted that this figure excluded turnover from small-scale farming, and was also inflated for companies headquartered in Kigali with revenue generated elsewhere in Rwanda. Official statistics classify economic activity as either "farm" or "non-farm", and Kigali accounts for 39 per cent of non-farm waged employees in the country.