GDP is a measure of the total value of goods and services produced in a country in a year. It can be calculated using the output, income, or expenditure methods. The output method sums the value added at each stage of production. The income method sums the incomes of households and companies. The expenditure method sums consumption, investment, government spending, and net exports. GDP is a key indicator of economic growth but does not account for income from abroad; GNP includes income earned domestically and abroad by a country's citizens and businesses. NNP further adjusts GNP by subtracting depreciation.