Lesson preview · Measuring Inflation
Measuring Inflation
~12 min · Free to read
A gallon of milk cost about 3.50 dollars in 2020. The same gallon today costs about 4.50. Your money buys less than it used to. Multiply that across every household budget and you have inflation.
Key Term
Inflation
A sustained rise in the average price level of goods and services. Note the word average — some prices rise, some fall, and inflation is the weighted-average move across the basket of things people actually buy.
Measuring inflation honestly is harder than it looks. You can’t just pick one good — milk inflation isn’t the inflation rate. You need a weighted basket. Two measures dominate.
Key Term
Consumer Price Index (CPI)
A fixed basket of about 80,000 specific goods and services that a typical urban household buys. The Bureau of Labor Statistics surveys prices each month and reports the weighted-average change. The ‘cost of living’ headline number in the news is almost always CPI.
Key Term
GDP Deflator
The price index built from GDP data — covered in Unit 2. Includes every good and service produced domestically, not just what consumers buy. Weights change automatically as the economy’s output mix shifts.
That gap is the whole point. Every household faces the same price changes, but because they spend their money differently, each one feels a different inflation rate. The headline CPI is just the rate for one made-up average household — and almost nobody is average.
CPI and the GDP deflator give different numbers because they cover different things. CPI includes imported consumer goods (a Korean TV, a Mexican avocado) and excludes capital goods. The GDP deflator includes domestic capital and government output and excludes anything not produced here. In most years the two agree to within a percentage point. In years with a big oil shock or a big trade-weighted dollar move, they can diverge sharply.
Key Term
Headline vs core inflation
Headline inflation includes everything in the CPI basket — including food and energy.
Core inflation strips food and energy out. The reason: food and energy prices bounce around with weather, oil shocks, and harvest cycles; the underlying trend in everything else is what central banks watch when setting policy.
Neither one is ‘the real number’ — they answer different questions. If you’re asking what households actually pay, use headline. If you’re asking where prices are heading, use core.
Trap · Common Misconception
Your personal inflation is not the CPI
If you mostly eat out and you don’t own a home, food and rent changes hit you twice as hard as the average household. CPI is built for the average household — single people, retired people, students, and rich people all have different baskets and different personal inflation rates. When your bill goes up faster than CPI, you’re not lying and CPI isn’t lying — your basket isn’t the average basket.
Trap · Warning
Three ways inflation numbers get spun
Watch for these moves in headlines and politicians’ speeches:
- Cherry-picked start dates. “Prices are 30% higher than in 2019!” might mean 6% per year averaged — actually fairly normal. The start year matters.
- Comparing nominal to real. “Wages rose 4%!” with inflation at 6% means real wages fell 2%. Always strip inflation out before declaring victory.
- Different baskets, same name. Some governments quietly change CPI methodology (different basket weights, different substitution rules) and report a lower number for the same underlying inflation.
Inflation rate — just the percent change in any price index from one period to the next.
Check yourself · no marks
An economy’s CPI was 230 last year and 241.5 this year. What is the inflation rate?
Commit to one first. Guessing and being wrong beats reading the answer cold.
Worked Example
A simplified CPI basket has three categories: housing (50% weight), food (20%), and other (30%). This year, housing prices rose 6%, food prices rose 12%, and other prices rose 2%. What is the headline inflation rate from this basket?
Practice · 1 / 4
What is the main difference between CPI and the GDP deflator?
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