What is ‘national income’ ? Explain the theoretical difficulties involved in the measurement of national income.

National income is the aggregate monetary -value of all final goods and services produced-in the, economy in a year.
Definitions of National Income:
Dr. Marshall's Definition                  
Marshall's definition represents a total value of production - it is from production end.
According to Marshall, "The labour and capital of a country acting on its natural resources produce annually a certain net aggregate of commodities, material and immaterial including services of all kinds.... This is the true net annual income or revenue of the country or national dividend."
Prof. Pigou's Definition
Pigou's definition of national income represents a receipts total production end.
According to Pigou, the national dividend is that part of the objective income of the community, including of course income derived from abroad, which can be measured in money:

Difficulties in the measurement of National Income :
The calculation of the national income of a country is a task full of difficulties and complexities. The following difficulties generally arise while estimating national income.
i)        Theoretical difficulties
ii)       Practical difficulties.
i)       Theoretical difficulties :
         This is also known as conceptual difficulties.
1)      Transfer payments:
Individuals get pension, unemployment allowance, but whether these should be included in national income is difficult problem. On one hand, these earnings are a part of individual income and, on the other, they are government expenditure. Therefore, these transfer payments are ignored from national income.

2)      Income of foreign firms:    
         According to IMF view-point, income of a foreign firm, should be included in the national income of the country, where the firm actually undertakes production work. However, profits earned by foreign firms are credited to the parent concern.
3)      Unpaid services:
National income is always measured in money, but there are a number of goods and services which are difficult to be assessed in terms of money. For example, painting as ahobby. by an individual, the bringing up of children by the mother, these services are not included in national income as remuneration is not given to them.
Also services of housewives and the services provided out of love, affection; mercy, sympathy and charity are not included in national income, as they are not paid for. By excluding all such services from it, the national income will work out to be less than what it actually is.
4)      Incomes from illegal activities:
Income earned through illegal activities such as gambling, black marketing, theft, smuggling etc., is not included in national income. Such goods and services do have value and meet the needs of the consumers. Thus to that extent national income is underestimated.
5)      Treatment of government sector: Government provides a number of public services like defence, public administration, law and order etc. Measuring the market value of such government services is difficult; as the real value of these services is not known, therefore it has become a convention to treat all such services as final consumption. Hence, it is included in national income.
6)      Production for self consumption:
Goods produced for self consumption such as food grains, vegetables and other farm products do not enter in the market. But the value of such goods should be estimated at the rate of market price that have been marketed and should be included in national income.
7)      Changing price levels:
The difficulty of price changes arise in the national income estimate, when the price level in the country rises, the national income also shows an increase even though the production might have fallen and when price level falls., National Income may show a decrease even though production may have increased.