It is measured by Real GDP, the reason is because you cant just say GDP. GDP consists of nominal and real GDP, nominal GDP does not include prices at different constants in other words it just uses one base price for all the different times, whereas real GDP consists of varying price levels at different times. Real GDP. The income method totals the various types of income earned by all the nation's households and firms in one year. The GDP is measured in order to provide a comparative tool with which to judge a national economy's annual rate of ...