The CPI Report: What It Measures and Why Markets Care

Once a month the U.S. Bureau of Labor Statistics (BLS) publishes the Consumer Price Index (CPI), and for a few minutes it can be the most important number in the world’s financial markets. Here is what the report contains and how to read it like a professional.

What CPI is

CPI measures the average change over time in the prices urban consumers pay for a fixed basket of goods and services. BLS collects prices for tens of thousands of items — food, rent, clothing, cars, medical care, airline fares — across many cities, and weights each category by how much households spend on it.

The headline figure most people quote is CPI-U, covering all urban consumers, who make up the large majority of the U.S. population.

Headline vs. core

• Headline CPI includes everything.
• Core CPI excludes food and energy.

Core is not “the inflation that matters” — people eat and buy fuel — but food and energy prices swing with weather, harvests and oil markets. Excluding them gives a clearer view of the underlying trend that monetary policy can influence. When headline and core point in different directions, economists usually put more weight on core for judging where inflation is heading.

Month-over-month vs. year-over-year

The report gives both:

• Year-over-year (y/y): prices compared with the same month a year earlier. This is the number in headlines (“inflation is 3.0%”).
• Month-over-month (m/m): the change from last month, seasonally adjusted.

Traders focus on the monthly core figure, because it is the new information. The annual figure is a 12-month window: it can fall simply because a large monthly increase from a year ago drops out of the calculation — a “base effect” — even if prices are rising quickly now.

A useful rule of thumb: a monthly core reading of about 0.2% annualizes to roughly 2.4%; 0.3% to about 3.7%; 0.1% to about 1.2%. Because the Fed’s target is 2%, the difference between 0.2% and 0.3% in a single month can move markets.

Shelter: the biggest piece

Housing costs, called shelter, are by far the largest component of CPI — roughly a third of the overall index and a larger share of core. Most of it is not house prices but rent and owners’ equivalent rent (OER), an estimate of what homeowners would pay to rent their own homes.

Shelter is measured from surveys of existing leases, so it reacts to market rents slowly, often with a lag of a year or more. That is why analysts also watch “core services excluding shelter” (sometimes called “supercore”), which tracks wage-sensitive services such as medical care, insurance and recreation.

CPI vs. PCE

The Federal Reserve’s 2% target is defined using a different index, the personal consumption expenditures (PCE) price index from the Bureau of Economic Analysis. PCE covers a broader range of spending (including what employers and governments pay on behalf of households, such as health care), weights shelter less heavily, and updates its weights as spending patterns change. PCE inflation usually runs somewhat below CPI inflation.

Because CPI comes out first, economists use it — along with the producer price index — to predict PCE before its release later in the month.

Why markets react

CPI moves markets because of what it implies for the Fed. A hotter-than-expected reading makes rate cuts less likely or rate hikes more likely, which tends to push Treasury yields up and stock prices down. A cooler reading does the reverse.

What matters is the surprise: the actual figure compared with the economists’ consensus forecast published in advance. A 3% inflation rate can be good news for markets if 3.2% was expected.

Reading the release in five minutes

1. Compare monthly core CPI with the consensus forecast.
2. Check the year-over-year trend for core and headline.
3. Look at shelter: is it slowing, as the lag from market rents would suggest?
4. Look at core goods (cars, furniture, clothing) and core services excluding shelter.
5. Note any one-off items, such as a jump in airline fares or car insurance, that might not repeat.

The full release, with detailed tables, is published on the BLS website at 8:30 a.m. Eastern time, usually in the middle of the month.