National income is defined as the value of all final goods and services produced by the normal residents of a country in a year. It is measured to understand the size and performance of a country's economy, trace economic growth trends, know the structure of the national income, and help formulate development plans and policies. National income is calculated using the production, income, and expenditure methods and aggregates at current and constant prices. It is important but does not fully reflect human development or income distribution.