Consumer Price Index

The Consumer Price Index (CPI) is the most widely cited inflation metric in the US. It measures the average change in prices that households pay for goods and services, from gasoline to haircuts to healthcare. The Bureau of Labor Statistics publishes CPI monthly, making it a timely inflation gauge.

What CPI measures

CPI tracks the prices consumers pay for:

• Food and beverages: Groceries, restaurant meals
• Housing: Rent, owners’ equivalent rent (imputed), utilities
• Transportation: Gasoline, car prices, insurance
• Healthcare: Medical services, prescription drugs
• Recreation and other: Clothing, entertainment, education

The “market basket” is fixed annually based on Consumer Expenditure Survey data. This is how much households actually spend on each category.

Headline versus core

Headline CPI:

• Includes all items, especially food and energy.
• More volatile due to oil price swings.
• What consumers actually pay at the grocery pump.
• Example: Oil spikes → headline CPI spikes 6 months later.

Core CPI:

• Excludes food and energy (volatile items).
• Smoother, better for identifying underlying trends.
• What the Federal Reserve watches most closely.
• Example: Oil spikes, but core CPI unaffected.

In 2021-22, headline CPI spiked due to energy prices; core CPI rose more moderately. The Fed focused on core as a better signal of demand-driven inflation.

Calculation methodology

CPI is calculated as:

CPI = Σ (Item weight × Item price change)

The BLS surveys ~15,000 retail locations monthly to get price quotes on thousands of items. These are weighted by household expenditure shares. The change from month-to-month is the monthly inflation rate; the change year-over-year is the annual inflation rate.

Example: If gasoline (4% of the basket) rises 15% in a month, it contributes 0.6 percentage points to CPI. If all other items are flat, CPI rises 0.6% that month.

Seasonal adjustment

CPI is seasonally adjusted (winter utilities costs more; summer clothing sales are deeper). Raw data would show inflation spiking in winter; seasonal adjustment irons this out.

Sticky-price CPI and trimmed-mean CPI

Alternative measures try to filter out noise:

• Sticky-price CPI: Only items that rarely change prices (rents, insurance, healthcare). Often turns faster than overall CPI.
• Trimmed-mean CPI: Removes the highest and lowest 10% price movers each month. Less volatile than headline, smoother than core.

The Fed watches all these variants to triangulate true underlying inflation.

CPI and real wages

The relationship between nominal wage growth and CPI inflation determines real wage growth:

Real wage growth = Nominal wage growth − CPI inflation

If wages rise 3% and CPI inflation is 4%, real wages fall 1%. This has been a concern in the 2021-25 period: nominal wage growth picked up, but CPI inflation eroded real gains initially.

Limitations of CPI

• It does not reflect quality improvements. A new car is vastly better than 1980s models, but CPI assigns a price based on specs that may not capture the improvement.
• Weighting is backward-looking. The basket is updated annually, so shifts in spending take time to filter in.
• It does not account for substitution. When beef prices spike, consumers switch to chicken; CPI assumes they keep buying beef.
• Owner-occupied housing is imputed. Homeowners’ “rent” is estimated (owners’ equivalent rent), which is noisy.

For these reasons, economists supplement CPI with alternatives like the PCE deflator.

CPI and markets

Markets react sharply to CPI releases. A “hotter than expected” CPI (higher than forecasts) typically:

• Weakens stocks (recession/rate-hike fears).
• Weakens bonds (yields rise as inflation expectations increase).
• Strengthens the dollar (higher interest rates attracted capital).

A “cooler than expected” CPI has the opposite effect.

See also