How do we measure the price level and the rate of inflation, and what causes it?
Topic 2.4 Price Indices and Inflation: define inflation and deflation, build and use the Consumer Price Index, calculate the inflation rate, and distinguish demand-pull from cost-push inflation.
A focused answer to AP Macroeconomics Topic 2.4, covering inflation and deflation, the Consumer Price Index and the market basket, calculating the CPI and the inflation rate, demand-pull versus cost-push inflation, and biases in the CPI, with full worked calculations.
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What this topic is asking
Topic 2.4 introduces the second major indicator: the price level and inflation. The College Board wants you to define inflation and deflation, to build and use the Consumer Price Index (CPI) from a market basket, to calculate the CPI and the inflation rate, and to distinguish demand-pull from cost-push inflation. This is a calculation-heavy topic that appears in both sections of the exam.
Inflation and deflation
Inflation is about the general price level, not a single good. A rise in one price (say, oil) is not inflation unless the average level of prices is rising. This is why economists use price indices that average across many goods.
The Consumer Price Index
Because the basket is fixed, the CPI isolates price changes from changes in what people buy. A CPI of 150 means prices on average are 50 percent higher than in the base year.
Calculating the inflation rate
If the CPI rises from 200 to 210, inflation is . Dividing by the new value is a common error that gives the wrong answer.
Demand-pull and cost-push inflation
The two causes of inflation differ in their source:
- Demand-pull inflation: "too much spending chasing too few goods." A rise in total (aggregate) demand pulls the price level up, usually when the economy is near capacity. Output and prices both rise.
- Cost-push inflation: rising production costs (for example, a spike in oil or wage costs) shift aggregate supply left, pushing prices up while output falls. The combination of rising prices and falling output (rising unemployment) is stagflation.
These two appear again in the aggregate demand and supply model, where demand-pull is a rightward AD shift and cost-push is a leftward AS shift, so learning the labels now pays off later.
Biases in the CPI
Because the basket is fixed, the CPI can overstate the true rise in the cost of living:
- Substitution bias: when one good gets more expensive, consumers switch to cheaper substitutes, but the fixed basket does not reflect this, overstating inflation.
- New goods and quality changes: the fixed basket is slow to include new products and improvements in quality, so it can misstate the real cost of a given standard of living.
Despite these biases, the CPI is the standard gauge of consumer-price inflation and the basis for cost-of-living adjustments, so understanding both how to compute it and where it errs is exactly the balanced view the exam wants. A strong free-response answer can compute the index and inflation rate, name the cause (demand-pull versus cost-push) by pointing to which curve shifts, and note at least one CPI bias. Keeping the arithmetic disciplined, build the basket cost each year, divide current by base and multiply by 100 for the index, then take the percentage change for inflation, prevents the most common slips, and tying each result back to purchasing power (higher prices mean each dollar buys less) shows you understand what the numbers mean rather than just how to crank them out.
Try this
Q1. Define inflation and explain how it affects the purchasing power of money. [2 points]
- Cue. Inflation is a sustained rise in the general price level; as prices rise, each unit of money buys fewer goods, so purchasing power falls.
Q2. A spike in global oil prices raises firms' costs across the economy. State which type of inflation this produces and what happens to output. [2 points]
- Cue. Cost-push inflation; aggregate supply shifts left, so prices rise while output falls (stagflation).
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2019 (style)1 marksMultiple choice. The Consumer Price Index rose from 200 to 210 over one year. The inflation rate for that year was (A) 10 percent. (B) 5 percent. (C) 4.8 percent. (D) 21 percent. (E) 2 percent.Show worked answer →
The answer is (B). The inflation rate is the percentage change in the price index: .
(A) wrongly uses the change of 10 as a percentage directly. (C) divides by the new value instead of the old. (D) and (E) are arithmetic errors. Always divide the change by the original (base) value.
AP 2021 (style)4 marksFree response. A simple economy's market basket contains 3 loaves of bread and 2 liters of milk. In the base year bread cost 1; in the current year bread costs 1.50. (a) Calculate the cost of the basket in each year. (b) Calculate the CPI for the current year (base year index = 100). (c) Calculate the inflation rate between the two years. (d) Identify whether a rise in production costs that shifts aggregate supply left produces demand-pull or cost-push inflation.Show worked answer →
A 4-point CPI calculation FRQ.
(a) Basket cost (1 point): base year . Current year .
(b) CPI (1 point): .
(c) Inflation rate (1 point): the base-year index is 100, so inflation .
(d) Type (1 point): a rise in production costs shifting aggregate supply left is cost-push inflation.
Markers reward both basket costs, the CPI of 150, the 50 percent inflation rate, and identifying cost-push inflation.
Related dot points
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- Topic 2.3 Unemployment: define the labor force and unemployment rate, calculate them, distinguish frictional, structural, and cyclical unemployment, and explain the natural rate and full employment.
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- Topic 2.5 Costs of Inflation: explain the real costs of inflation, distinguish anticipated from unanticipated inflation, and identify how inflation redistributes income between borrowers, lenders, and people on fixed incomes.
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- Topic 2.6 Real versus Nominal GDP: distinguish nominal from real GDP, use the GDP deflator to convert between them, and explain why real GDP is the correct measure of output growth.
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Sources & how we know this
- AP Macroeconomics Course and Exam Description — College Board (2023)