There are at least two or three different ways of calculating these numbers. The '''expenditure approach''' determines aggregate demand, or Gross National Expenditure, by summing consumption, investment, government expenditure and net exports. On the other hand, the '''income approach''' and the closely related '''output approach''' can be seen as the summation of consumption, savings and taxation. The three methods must yield the same results because the total expenditures on goods and services (GNE) must by definition be equal to the value of the goods and services produced (GNP) which must be equal to the total income paid to the factors that produced these goods and services (GNI). (GNP=GNI=GNE by definition)
In actual fact, there will be minor differences in the results obtained from the various methods due to changes in inventory levels. This is because goods in inventory have been produced (and therefore included in GDP), but not yet sold (and therefore not yet included in GNE). Similar timing issues can also cause a slight discrepancy between the value of goods produced (GDP) and the payments to the factors that produced the goods (particularly if inputs are purchased on credit).