Inflation, the general rise in the prices of goods and services, is important to policymakers for several reasons. First, rising inflation is unpopular with the public, in part because some households are more adversely affected by inflation than others. Second, high or rising inflation can reduce productivity by distorting price signals, so that it is hard for businesses to tell if prices are changing in relative terms, and by individuals wasting resources in order to maintain the purchasing power of their wealth. Finally, inflation plays a key role in macroeonomic stabilization policy. Changes in inflation often indicate changes in the business cycle—rising inflation is often a sign that the economy is overheating and falling inflation is a sign that the economy is sluggish. The Federal Reserve (Fed) is mandated to keep inflation low and stable, and alters interest rates in order to do so. In recent years, the Fed has focused attention on the core rate of inflation, a measure of inflation that excludes food and energy prices, in explanations of its policy decisions. For example, in July 2007, the third sentence of the 10-sentence Federal Open Market Committee statement summarizing the committee's policy decision read, "Readings on core inflation have improved modestly in recent months." In Fed Chairman Ben Bernanke's July 2007 testimony to Congress, he stated that "Food and energy prices tend to be quite volatile, so that, looking forward, core inflation...may be a better gauge than overall inflation of underlying inflation trends." When core inflation approached 3% in 2006, Chairman Bernanke said that it had "reached a level that, if sustained, would be at or above the upper end of the range that many economists, including myself, would consider consistent with price stability...." This report defines core inflation, reviews recent trends, and analyzes the advantages and drawbacks of using core inflation. No official measure of "inflation" exists. Inflation is measured as the percent change in a price index. Several indices track price changes, with each data series measuring something different. The most commonly cited measure of inflation is the percent change in the consumer price index (CPI) . This index measures the price of a basket of consumer goods and services that is representative of overall consumer purchases in urban areas. When food and energy prices are omitted from the CPI, the remaining basket is commonly referred to as the core CPI . The overall measure of CPI, which includes food and energy, is often referred to as the headline CPI . Another common measure of inflation is the percent change in the GDP (gross domestic product) price deflator , which is used to transform nominal GDP into real GDP. Since the GDP deflator is based on the prices of all goods and services in the economy, it is a broader measure of inflation than the CPI. A subset of the GDP deflator that is conceptually similar to the CPI, but includes more items and areas, is the personal consumption expenditures (PCE) price deflator ; for technical reasons, the Fed sometimes prefers this measure to the CPI in their analyses. Core measures of the GDP and PCE deflators are also available. Conceptually, core inflation could be any measure of inflation that attempts to strip out price volatility, but the most common definition of core strips out only two particularly volatile categories of goods, food and energy. The four most volatile items in the CPI are all food or energy products. The standard deviation of energy prices is estimated to be 12 times higher than overall inflation. Omitting food and energy prices from the CPI is not a trivial modification—food and beverages accounted for 15% of the headline CPI basket, and energy accounted for an additional 9% in 2006. While excluding food from core inflation has become conventional, it may no longer be warranted. The volatility of food has decreased significantly since the 1970s. Until 2007, the recent divergence between headline and core inflation was driven by energy prices. In 2007, food prices rose rapidly—it is too soon to tell whether this development marks a renewed period of persistent volatility. If food prices are no longer volatile, then policymakers may be losing useful information by omitting them. In recent years, headline inflation has typically outpaced core inflation, as seen in Figure 1 , because of the rapid rise in energy prices. In 2007, headline inflation was also driven up by a 3.9% increase in food prices. The difference between core and headline has not always been trivial—from 2003 to 2006, core inflation was 0.9 percentage points lower than headline. Considering that the Fed judges 2% inflation to be on the low side and 3% inflation on the high side, the definition used in these years would have arguably strongly colored their policy stance. The difference between core and headline inflation over this period was overwhelmingly the result of energy prices, which rose by an average of 12.8% a year as measured by the CPI. When comparing purchasing power over two time periods, headline inflation is the relevant measure. Comparisons over time of wages, wealth, rates of return, government transfers such as Social Security payments, and so on should all use a headline measure of inflation, because all of these concepts depend on a broad measure of inflation. For example, adjusting household income by core inflation would not be useful since food and energy consumption account for about one-quarter of average household expenditures. Similarly, government programs and parts of the tax code that are adjusted for inflation are based on headline inflation. Economic growth is also calculated by first adjusting GDP by headline inflation. Core inflation is used by policymakers for the reason offered by Chairman Bernanke in the introduction—policymakers are most concerned about the future path of inflation, and current core inflation data may give better information than current headline data about future headline inflation. Headline inflation often does not have good predictive power over short-time periods because food and energy prices are so volatile. For example, the monthly headline inflation rate varied between -6.3% and 7.5% in 2006 at annualized rates, whereas the core rate varied between 1.2% and 3.6%. Policymakers are concerned with future inflation because of lags between a change in policy and its effect on the economy. In essence, it is already too late for policy to influence current inflation, a policy change today can only affect future inflation. Theoretically, short-term changes in inflation can be caused by the supply-side or demand-side of the economy. When rising inflation is demand-driven, it means that spending is growing too quickly in the overall economy, and production cannot keep pace. This phenomenon is captured in the famous saying "too much money chasing too few goods." The Fed's task is to counteract this by raising interest rates in order to reduce the growth rate of interest-sensitive spending. Likewise, if spending is rising too slowly, inflation will fall, which the Fed can counteract by reducing interest rates. In the short run, the overall inflation rate can also be affected by sharp price changes of individual goods caused by supply shocks. For example, bad weather can drive up food prices or a reduction in the oil supply can drive up energy prices. Since these supply shocks are temporary, they should not have any lasting effect on inflation (holding aggregate spending constant), in which case they can be ignored by policymakers. In the long run, price shocks on the supply side should cancel each other out (since, across all goods, there will be an equal number of positive and negative surprises), and average inflation should be completely demand driven. Ideally, policymakers would like to be able to identify whether any change in inflation was demand-driven or supply-driven. Unfortunately, there is no straightforward way to do this, so they have commonly used core inflation as a proxy for demand-driven inflation, reasoning that food and energy are two sectors of the economy that are most susceptible to supply shocks. Furthermore, policymakers are particularly concerned with inflationary expectations, and a rising core rate may be a better sign than rising headline that inflationary expectations have risen. Relying on core inflation for policymaking has its drawbacks, however. There is no inherent reason that changes in food and energy prices cannot be caused by changes in aggregate demand. For example, rapid spending growth could push up energy prices if supply does not rise in response. In fact, an argument has been made that a change in aggregate demand would first show up in price changes of goods that have flexible pricing, such as commodities that are traded on financial markets where prices change continually to clear the market. Both energy and basic foodstuffs are traded on financial markets, although the CPI measures final food and energy products, not basic commodities. Furthermore, a rise in the price of any one good need not lead to a change in inflation if the prices of other goods fall to offset it. Technically, if a rise in one price leads to a rise in overall inflation, it must be because of some accommodation on the Fed's part (because it did not raise interest rates enough to induce other prices to fall). Most economists believe that some accommodation to relative price changes is desirable because it reduces the volatility of economic growth, whereas zero accommodation could lead to needless disruptions in economic activity. For example, Fed Governor Frederic Mishkin used the Fed's macro model of the U.S. economy to show that when the Fed reacts to changes in headline inflation instead of core inflation, future inflation will be slightly less volatile, but unemployment will be significantly more volatile. But if the Fed accommodates a rise in the price of one good too much, then the price of all goods could start rising. In other words, a rise in headline inflation could feed through to higher core inflation. This scenario occurred in the 1970s where rising energy prices resulted in a rise in total inflation. In scenarios like this one, a focus on core inflation could forestall a needed policy change until it is too late.