National income:

The national income or product or expenditure provides a measure of total value at factor cost of final goods and services, which are available either for consumption or for addition to wealth.
 
GDP is the monetary value of final goods and services produced in an economy (for example within the four walls of Ghana) in a given time period, say one year.

National Income estimates are very useful to economists, policy makers, business people and investors. However, some problems are faced in accounting for national income.