To compute the national income of a country is difficult problem because of following difficulties.
Measured in Money
National income is always measured in money but there are a number of goods and services which are complicated to be evaluate in terms of money e.g. painting as a hobby by an individual, the bringing up of children by the mother.
The greatest difficulty in calculating the national income is of double counting, which arises from the failure to distinguish properly between a final goods and services are taken into account, and that is not so easy task.
Income earned though illegal activities such as gambling, or sale of intoxicants etc. not incorporated in national income .Such good and services do have value and meet up the needs of the customers. But by leaving them out, the NI works out to less than the actual.
Then there occur the obscurity of including transfer payments in the national income. Individuals get unemployment allowance and interest on public loans, but whether these should included in NI is a difficult problem. On the one hands, these earnings are a part of individual income and on the other, they are government expenditure. To evade this complexity, these are subtract from national income.
Capital Gains or Losses
Capital gains or losses which accrue to property owners by increase or decrease in the market value of their capital assets or change in demand are excluded from the G.N.P became such changes do not result from current economics activities.
All Inventory Changes
All inventory changes whether negative or positive included in the G.N.P.
When we deduct capital decrease from G.N.P, the resulting measure is N.N.P.
Another difficulty in calculating national ,income is that of price changes which fail keep stable the measuring rod of money for national income .When the price level in the country rises, the national income also shows an increase even though the production might have fallen.On the contrary , with a fall in price level ,the NI shows a decline over though the production might have gone up.
Under Estimation Of Real National Income
Moreover, the calculation of national income in terms of money is under-estimation of real national income. It does not include the leisure foregone in the process of production of commodity.
In calculating national income ,a good number of public services also taken which cannot estimated corrected . How should there public and military services estimated? In the days of war, the force are active, but during peace they rest in cantonments.