A price index is a measure of the cost of a fixed basket of goods and services in a given year relative to a base year, used in AP Macro (Topic 2.4) to calculate the inflation rate and convert nominal variables like wages and GDP into real ones.
A price index turns "prices went up" into an actual number. You take a fixed basket of goods and services, find what it costs in some year, divide by what it cost in the base year, and multiply by 100. The base year always equals 100, so an index of 210 means the basket costs 110% more than it did in the base year.
In AP Macro you work with two price indices. The Consumer Price Index (CPI) tracks the basket a typical urban household buys, and per the CED it measures the change in income a consumer would need to maintain the same standard of living as prices change. The GDP deflator covers everything produced domestically. Either one lets you do the two jobs the exam cares about. First, calculate the inflation rate as the percentage change in the index between two years. Second, deflate nominal values into real values so you can compare purchasing power across time. (The Producer Price Index exists, but calculating PPI is explicitly excluded from the course.)
Price indices live in Topic 2.4 of Unit 2 (Economic Indicators and the Business Cycle) and anchor four learning objectives. You define CPI, inflation, deflation, and disinflation (AP Macro 2.4.A), explain how indices compare nominal variables over time (2.4.B), actually calculate CPI, inflation rates, and real variables (2.4.C), and identify CPI's shortcomings like substitution bias (2.4.D). This matters beyond Unit 2 because the entire rest of the course runs on the nominal-versus-real distinction. Real GDP, real interest rates, and real wages all depend on a price index doing the adjusting. If you can't work an index, half of macro stops making sense.
Keep studying AP Macroeconomics Unit 2
Consumer Price Index (CPI) (Unit 2)
CPI is the specific price index the exam tests most. It uses a fixed consumer basket, which is also its weakness, since the fixed basket creates substitution bias and makes CPI overstate true inflation (EK MEA-1.G.1).
Inflation Rate (Unit 2)
The inflation rate is the percentage change in a price index between two periods. The index is the level; inflation is how fast that level is climbing.
Real variables (Units 2-4)
Dividing a nominal value by a price index (times 100) gives you the real value. This one move converts nominal GDP to real GDP, nominal wages to real wages, and nominal interest rates to real ones via the Fisher relationship.
Central Bank (Units 4-5)
Central banks set monetary policy based on what price indices show. When the index is rising too fast, contractionary policy follows, so the humble index from Unit 2 is the trigger for everything the Fed does later in the course.
Price index questions show up mostly as multiple-choice calculations and concept checks. Expect stems asking what is or isn't included in the CPI basket, what counts as a limitation of CPI (substitution bias is the go-to answer), and number-crunching like this one: if CPI was 200 in 2020 and 210 in 2021, a $50,000 salary in 2021 is worth $50,000 × (200/210) ≈ $47,619 in 2020 dollars. You should be able to compute an index value, turn two index values into an inflation rate, and deflate a nominal variable. On FRQs, price indices usually appear in service of bigger questions about inflation, unemployment, and policy responses rather than as standalone prompts, so treat the index math as a tool you grab quickly, not a topic you write paragraphs about.
A price index is a level; the inflation rate is a rate of change. An index of 210 doesn't mean 210% inflation. It means prices are 110% above the base year. Inflation between two years is the percentage change between the two index values, so going from 200 to 210 is (210−200)/200 = 5% inflation. Also watch the deflation/disinflation trap. The index falling means deflation, while the index rising more slowly than before means disinflation.
A price index measures the cost of a fixed basket of goods and services in a given year relative to a base year, and the base year is always set to 100.
The inflation rate equals the percentage change in a price index, such as CPI or the GDP deflator, between two periods.
To convert a nominal value to a real value, divide by the price index and multiply by 100 (or use the ratio of the base-year index to the current index).
The CPI overstates true inflation because of substitution bias, since its fixed basket ignores consumers switching to cheaper substitutes when relative prices change.
An index of 210 means prices are 110% higher than the base year, not that inflation is 210%, and a falling index means deflation while slower index growth means disinflation.
Calculating the Producer Price Index is excluded from the AP Macro exam, so focus on CPI and the GDP deflator.
It's a measure of the cost of a fixed basket of goods and services in a given year relative to a base year, where the base year equals 100. AP Macro uses it to calculate the inflation rate and convert nominal variables into real ones (Topic 2.4).
No. The price index is a level, and the inflation rate is the percentage change in that level between two periods. If CPI goes from 200 to 210, inflation is 5%, not 210%.
CPI tracks a fixed basket of goods a typical consumer buys, including imports, while the GDP deflator covers all goods and services produced domestically. Both can be used to calculate an inflation rate per EK MEA-1.F.3.
Because of substitution bias (EK MEA-1.G.1). The CPI uses a fixed basket, so it ignores that consumers switch to cheaper substitutes when prices rise, making measured inflation higher than the true cost-of-living increase.
Divide the nominal value by the index and multiply by 100, or multiply by the ratio of the old index to the new one. For example, a $50,000 salary in 2021 with CPI at 210 is worth $50,000 × (200/210) ≈ $47,619 in 2020 dollars when 2020's CPI was 200.
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