Inflation is not one number: CPI, core inflation and PCE

macroPublished 19 October 20262 min readmacro

Key points

• A headline index includes food and energy. A core index removes a stated set of volatile items. Neither is the true inflation.
• CPI and PCE use different baskets and different weights. A gap between them is a measurement fact, not a contradiction to be averaged away.
• The arithmetic illustration below is hypothetical. It is not a BLS or BEA print.
• The September 2026 FOMC statement said inflation was elevated. It did not quote CPI or PCE.

What is going on

A consumer price index follows a basket of goods and services households buy. A personal consumption expenditures price index follows the prices of what is consumed in the national accounts, with weights that can differ, including items paid for on behalf of households. 'Core' means a published exclusion, often food and energy, not a deeper truth. Producer prices sit earlier in the chain. Expectations, whether from surveys or from market prices, are not the same object as a receipt. The existing note on several ways to measure inflation sets out that map. This note stays on the US household pair and on the arithmetic people use to turn an index into a rate.

Why it matters

The Federal Reserve's 16 September 2026 statement refers to inflation and to a 2 percent goal. It does not say which index printed which rate that month. Importing a CPI figure from memory, or from a different month, would make the statement look more precise than it is. Policy can respond to a series the reader is not watching. Markets can respond to the surprise versus what was expected, which is a third number, also absent here.

What to watch

Read the index name, the seasonal adjustment, the base and whether the change is from a month ago or from a year ago. A percentage-point gap between two inflation rates is not a percent change in inflation. If CPI is 3 percent and PCE is 2 percent, PCE is not 'one percent lower' in the sense of a ratio. It is one percentage point lower. The next section shows the index arithmetic on made-up levels so the language has an object.

Risks

Base effects can make a yearly rate fall because a large monthly rise is dropping out of the window, even when the latest month was not soft. Energy can move the headline while the rest of the basket is quiet, or the reverse. A hypothetical example that is later quoted as a fact would undo the point of the example.

A labelled illustration, not a release

Suppose an index stands at 100 in a base month and at 103 a year later. The change is 3 percent. Suppose the next month's index is 103.3. The monthly change is about 0.3 percent. Compounding twelve such months is not the same operation as comparing the index with its level twelve months earlier, and neither operation is a forecast. These levels were chosen to make the arithmetic visible. They were not taken from a statistical office.