What is CPI?

The Consumer Price Index (CPI) is a measure of the average change over time in the prices paid by urban consumers for a market basket of consumer goods and services. Published monthly by the Bureau of Labor Statistics (BLS), it is the most widely followed gauge of inflation in the United States and a key input into Federal Reserve monetary policy decisions.

The CPI basket contains over 200 categories of goods and services, organized into eight major groups: food, housing, apparel, transportation, medical care, recreation, education and communication, and other goods and services. Each category is weighted according to its share of consumer spending. Shelter costs (rent and owners' equivalent rent) represent roughly one-third of the index, making them the single largest component.

There are several variants of CPI. Headline CPI includes all items and captures the full cost-of-living experience. Core CPI strips out food and energy prices, which are volatile and often driven by supply shocks rather than underlying demand conditions. The Fed tends to focus more on PCE (Personal Consumption Expenditures) for its inflation target, but CPI remains the more market-moving release due to its earlier publication date and wider public recognition.

CPI data is released on the second or third Tuesday of each month and is one of the most market-moving economic reports. Higher-than-expected readings typically push bond yields higher and equity prices lower, as traders price in more aggressive Fed tightening. Lower-than-expected readings have the opposite effect, loosening financial conditions.

It is important to note that CPI has limitations. It uses fixed-weight methodology that may not capture real-time substitution effects, and its shelter component (based on rent surveys and owners' equivalent rent) lags actual housing market conditions by roughly 12-18 months. Understanding these measurement nuances is important for interpreting CPI data correctly and anticipating where inflation is actually headed versus where the official statistics say it has been.