<P> Note that all three counting methods should in theory give the same final figure . However, in practice minor differences are obtained from the three methods for several reasons, including changes in inventory levels and errors in the statistics . One problem for instance is that goods in inventory have been produced (therefore included in Product), but not yet sold (therefore not yet included in Expenditure). Similar timing issues can also cause a slight discrepancy between the value of goods produced (Product) and the payments to the factors that produced the goods (Income), particularly if inputs are purchased on credit, and also because wages are collected often after a period of production . </P> <P> Gross domestic product (GDP) is defined as "the value of all final goods and services produced in a country in 1 year". </P> <P> Gross national product (GNP) is defined as "the market value of all goods and services produced in one year by labour and property supplied by the residents of a country ." </P> <P> As an example, the table below shows some GDP and GNP, and NNI data for the United States: </P>

Where do government economists get the data for the national income accounts