JentooMacroeconomics

AP® Macroeconomics · Unit 2 · Topic 2.4

Why the CPI Overstates Inflation

The CPI tracks a fixed basket of goods, so it misses the ways people actually dodge price increases and the ways products improve, which makes measured inflation run higher than true inflation.

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The CPI is built from a basket of goods fixed at some base period. Real households don't shop that way. When beef gets expensive, people buy more chicken and less beef. Their actual cost of living rises less than the fixed basket implies, because the fixed basket assumes they keep buying just as much beef as before. This is substitution bias: the measured index keeps pricing out a basket people have already abandoned, so it climbs faster than people's real spending does.

Quality change causes a second kind of overstatement. If a laptop costs 900 dollars this year and 900 dollars last year but is twice as fast with double the storage, the CPI sees a 0 percent price change. But you are getting far more computing power for the same money, so the true price of a unit of computing power fell. Recording this as unchanged overstates how much more it costs to live. The same logic applies to new products entering the market: when something like a smartphone first appears, it takes time for it to enter the basket at all, so the index misses the value it adds to consumers' lives in the meantime.

These biases matter because Unit 2 treats CPI as the measure of the price level driving real variables like real wages and real GDP. If CPI runs hot, real wage calculations understate how well-off workers actually are, and inflation-adjusted comparisons across years look worse than reality. Policymakers who rely on CPI to set cost-of-living adjustments or judge the Fed's inflation target need to know the number they're reading is a biased estimate, not a perfect one.

Key terms

Substitution bias
The overstatement of inflation that happens because a fixed basket ignores consumers shifting toward relatively cheaper goods.
Quality bias
The overstatement of inflation that happens when product improvements at a stable price are recorded as no price change at all.
New product bias
The overstatement of inflation caused by delays in adding newly invented goods to the CPI basket, missing the value they add.
Fixed basket
The unchanging set of goods and quantities the CPI prices every period to track cost of living.

Watch it explained

  1. How it worksHow does substitution bias push the measured inflation rate over the true one?
  2. Common mistakeWhy isn't a laptop that costs the same but runs twice as fast a price change of zero?
  3. Going furtherWhat happens to the measured index when a brand new kind of product appears?

Check yourself

Your turn1 of 2
The price of beef rises sharply, so consumers switch to buying more chicken instead, keeping their actual grocery spending increase modest. But the CPI's fixed basket still assumes they buy the same amount of beef as before. What effect does this have on measured inflation?

End of topic.

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