GDP is measured over a specified period of time. In principle, that time period could be anything—a week, a month, a quarter (three months), or a year. In the United States and many other countries, GDP is measured on a quarterly basis. However, it is typically reported on an annual basis. In other words, government statisticians might measure GDP for the first three months of 2012 and find that it was $4 trillion. That is, over that three-month period, $4 trillion worth of goods and services was produced. The number would typically be reported as “$16 trillion on an annual basis.”
It does not make any sense to talk about US GDP at the instant the clock strikes noon on February 29, 2012. The amount of GDP produced at any instant of time is, for all intents and purposes, zero. Instead, we think of GDP as a flow. We can count the number of pizzas produced only if we specify some interval of time. Other variables can be sensibly measured even at a given instant. For example, we could—in principle at least—count the number of pizza ovens in existence at any given time. The number of pizza ovens at a point in time is an example of a stock.
The requirement that we count goods and services produced in a certain period means that we should also ignore the resale of goods produced in earlier periods of time. If a construction company builds a new house and sells it to you, the production of that home is counted as part of GDP. By contrast, if you buy a house that is 10 years old, the sale of that house is not counted in GDP. (However, if you employed a real estate company to find the old house for you, payment to that company would be included as part of GDP.) In the same way, if you purchase a used textbook that was produced 3 years ago, that purchase is not counted in GDP.
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