Measuring Real GDP Using Nominal GDP, GDP Price Index, GDP Deflator

A primary benefit of measuring the Gross Domestic Product (GDP) is that it can show the growth of the economy over time, or its lack thereof. However, GDP as measured by current prices does not measure the growth of real GDP, since prices depend on the money supply, which varies independently of GDP from year to year. Nominal GDP is the GDP measured by actual prices, which are unadjusted for inflation. Real GDP measures output in constant dollars, so that the economic output of one year can be accurately compared to a previous year. Since prices change from year to year, GDP would change from year to year even if the real GDP did not change. Hence, there must be some adjustment in GDP to reflect the change in prices. Rising prices inflate GDP while falling prices deflate GDP. Therefore, to obtain real GDP, the operation must be reversed. Since prices usually rise, GDP is deflated by the amount of the inflation to arrive at real GDP. Hence, it is often called the GDP deflator.

The GDP deflator is based on a GDP price index and is calculated much like the Consumer Price Index (CPI), based on data collected by the government. Although the GDP index is much like the CPI, it covers many more goods and services. The CPI only covers consumer goods and services, while the GDP index also covers capital goods, purchases by the government, and goods and services that are traded worldwide.

The Bureau of Economic Analysis (BEA), along with the Bureau of Labor Statistics (BLS), compiles the GDP accounts. BEA is an agency of the United States Department of Commerce that produces and distributes economic statistics about governments, businesses, households, and individuals. The most important of these statistics is the National Economic Accounts, which provides statistics on the production, distribution, and the use of economic output, of which the GDP is the most prominent. Most of the information for calculating the GDP accounts for consumption and investment comes from the United States Census Bureau. Information about government purchases comes from the United States Office of Personnel Management about wages and benefits, which is the single largest expenditure by the federal, state, and local governments. The Census Bureau also provides some of this information. Most of the information on net exports comes from the United States Customs Service. The Bureau of Labor Statistics provides most of the statistics concerning labor, which is the major input to GDP.

To determine the value of the GDP deflator, a GDP price index must be constructed that shows how much prices have changed from year to year for a representative sample of all products and services. The relative weights of various goods and services in the GDP price index are adjusted annually, unlike the CPI, which is a fixed weight price index for a market basket whose composition is only updated occasionally by the BLS.

A price index is a measure of how much prices have changed in any given year as compared to a base year:

Price Index in a Given Year =Price of Representative Market Basket in Specific Year
Price of Representative Market Basket in Base Year
× 100

The multiplication by 100 gives a nice round number, especially for reporting. However, to determine real GDP, the nominal GDP is divided by the price index divided by 100.

To simplify comparisons, the value of the price index is set at 100 for the base year. Previous to the base year, prices were generally lower, so those GDP values must be inflated to compare them to the base year. When prices are less in any given year than they were in the base year, then the price index will be less than 100, so that when real GDP is calculated by dividing the nominal GDP by the price index, it will be greater than the nominal GDP.

Another method of calculating real GDP is to enumerate the volume of output, then multiplying that volume by the prices of the base year. So if a gallon of gas cost $2 in the year 2000, and the United States produced 10,000,000,000 gallons, then these values can be compared to a later year. For instance, if the United States produced 15,000,000,000 gallons of gasoline in the year 2010, then the real increase in GDP with respect to gasoline could be calculated by simply multiplying the 15 billion by the 2000 price of $2 per gallon. The price index can then be calculated by dividing the nominal GDP by the real GDP. So if gasoline was $3 per gallon in 2010, then the price index would be equal to 3 divided by 2 multiplied by 100 which is equal to 150.

Of course, there are many complexities to calculating real GDP by either method. Statisticians must necessarily use assumptions about the proportion of each type of goods and services purchased during a given year. If you would like to dive into the details of calculating this chain-type annual-weights price index, be my guest: Box: Basic Formulas for Calculating Chain-Type Quantity and Price Indexes.

Below is a graph showing the real GDP for the United States from 1990 - 2010, with the base year of 2005:Graph showing the real GDP for the United States from 1990 - 2010, with the base year of 2005.