Gross Domestic Product

The total amount of goods and services produced, or the total amount of income earned, or the total amount of expenditure undertaken, all tell us something about the overall performance of the economy in providing resources for the members of the society. If we could measure these totals, we could examine whether the resources available to the residents of a country were changing through time, or try to compare the country’s situation with that in other nations. Of course, there is such a measure. It is called GDP and it is a key way in which we try to monitor the performance of an economy.

Here is a definition of GDP:

Gross domestic product (GDP) is the market value of all final goods and services produced within a country in a given period of time. Let's consider each phrase in this definition with some care.

"GDP is a Market Value..." You have probably heard the adage, “You can't compare apples and oranges”. Yet GDP does exactly that. GDP adds together many different kinds of products into a single measure of the value of economic activity. To do this, it uses market prices. Because market prices measure the amount people are willing to pay for different goods, they reflect the value of those goods. If the price of an apple is twice the price of an orange, then an apple contributes twice as much to GDP as does an orange.

"Of All..." GDP tries to be comprehensive. It includes all items produced in the economy and sold legally in markets. GDP measures the market value of all the goods. GDP also includes the market value of the housing services provided by the economy's stock of housing. For rental housing, this value is easy to calculate – the rent equals both the tenant's expenditure and the landlord's income. Yet many people own the place where they live and, therefore, do not pay rent. The government includes this owner-occupied housing in GDP by estimating its rental value. That is, GDP is based on the assumption that the owner, in effect, pays rent to himself, so the rent is included both in his expenditure and in his income.

There are some products, however, that GDP excludes because measuring them is so difficult. GDP excludes items produced and sold illicitly, such as illegal drugs. It also excludes most items that are produced and consumed at home and, therefore, never enter the marketplace. Vegetables you buy at the grocery store are part of GDP; vegetables you grow in your garden are not.

"Final..." When International Paper makes paper, which Hallmark then uses to make a greeting card, the paper is called an intermediate good, and the card is called a final good. GDP includes only the value of final goods. The reason is that the value of intermediate goods is already included in the prices of the final goods. Adding the market value of the paper to the market value of the card would be double counting. That is, it would (incorrectly) count the paper twice.

An important exception to this principle arises when an intermediate good is produced and, rather than being used, is added to a firm's inventory of goods to be used or sold at a later date. In this case, the intermediate good is taken to be “final” for the moment and its value as inventory investment is added to GDP. When the inventory of the intermediate good is later used or sold, the firm's inventory investment is negative, and GDP for the later period is reduced accordingly.

"Goods and Services..." GDP includes both tangible goods (food, clothing, cars) and intangible services (haircuts, housecleaning, doctor visits). When you buy a CD by your favorite singing group, you are buying a good, and the purchase price is part of GDP. When you pay to hear a concert by the same group, you are buying a service, and the ticket price is also part of GDP.

"Produced..." GDP includes goods and services currently produced. It does not include transactions involving items produced in the past. When General Motors produces and sells a new car, the value of the car is included in GDP. When one person sells a used car to another person, the value of the used car is not included in GDP.

"Within a Country..." GDP measures the value of production within the geographic confines of a country. When a Canadian citizen works temporarily in the United States, his production is part of U.S. GDP. When an American citizen owns a factory in Haiti, the production at his factory is not part of U.S. GDP. (It is part of Haiti's GDP.) Thus, items are included in a nation's GDP if they are produced domestically, regardless of the nationality of the producer.

"... In a Given Period of Time." GDP measures the value of production that takes place within a specific interval of time. Usually that interval is a year or a quarter (three months). GDP measures the economy's flow of income and expenditure during that interval.

When the government reports the GDP for a quarter, it usually presents GDP “at an annual rate”.

In addition, when the government reports quarterly GDP, it presents the data after they have been modified by a statistical procedure called seasonal adjustment. The unadjusted data show clearly that the economy produces more goods and services during some times of year than during others. (As you might guess, December's Christmas shopping season is a high point.) When monitoring the condition of the economy, economists and policymakers often want to look beyond these regular seasonal changes. Therefore, government statisticians adjust the quarterly data to take out the seasonal cycle. The GDP data reported in the news are always seasonally adjusted.

Economists distinguish between nominal GDP and real GDP. Nominal GDP is the value of all final goods based on the prices existing during the time period of production. Real GDP is the value of all final goods produced during a given time period based on the prices existing in a selected base year. In other words, the prices in the base year provide the basis for comparing quantities in different years.

As we have just seen, nominal GDP reflects both the prices of goods and services and the quantities of goods and services the economy is producing. By contrast, by holding prices constant at base-year levels, real GDP reflects only the quantities produced. From these two statistics, we can compute a third called the GDP deflator, which reflects the prices of goods and services but not the quantities produced. The GDP deflator is calculated as follows: