The Fed swapped its inflation yardstick from CPI to PCE, and accuracy was not the only reason

The US personal consumption expenditures price index fell 0.1% in June from the month before. That is the first monthly decline in this index in six years. Against a year earlier it ran 3.7%, down from 4.1% in May. The biggest single mover was energy goods, including gasoline, which fell 9.2%.

Inflation rate fell by 0.1% in June, first decrease in six years (UPI, 2026-07-30)

출처: upi.com

UPI reports that the 9.2% drop in energy goods was the largest since August 2022. What pushed the index negative for a month, in other words, was energy rather than prices in general. The reason markets cheered the number sits two days earlier, at the Fed meeting. Rates were held 9 to 3, and all three dissents were for a hike rather than a cut. Data showing prices cooling takes the wind out of the side arguing for higher rates.

Why the Fed does not watch the CPI

Here is the part that snags. The number we call inflation in the news is usually the consumer price index, the CPI. But the yardstick the Fed measures its 2% target against is not the CPI. It is the personal consumption expenditures price index, the PCE. The two are built differently. The CPI fixes more than a hundred thousand items into a basket and prices that basket. The PCE keeps changing what is in the basket according to what people actually bought. When beef gets expensive, households buy more chicken. A fixed basket cannot follow that move, so it charges the full beef increase to the index. That failure to see shoppers switching to whatever got cheaper, which makes measured inflation run high, is called substitution bias.

Why this statistic was a political problem

The homework in front of Congress in 1995 was the deficit. More than half of federal spending was mandatory, programs like Social Security, and both those increases and the income tax brackets were indexed to the CPI. Indexed means one CPI print moves spending and tax revenue at the same time. So a finding that the CPI overstated inflation changed the arithmetic. It would slow the growth of outlays without anyone having to vote to cut pensions. The commission Congress set up delivered its verdict on December 4, 1996. The CPI was overstating inflation by about 1.1 percentage points a year, and substitution bias was the largest single piece of it.

The report itself spells out why a statistic was a money question. If the CPI overstated the cost of living by one percentage point a year for a decade, the report calculated, the resulting addition to the deficit would reach roughly $135 billion by 2006, making it the fourth largest federal program behind Social Security, health care and defense. The Fed's official yardstick changed from CPI to PCE in February 2000.

Does that yardstick really read lower

The effect of swapping yardsticks is not the kind of thing a single month confirms. Averages running lower and this month running lower are different claims, and this month was inverted.

How we will know

In the end there is one question. Is this cooling the start of a trend, or a one month energy effect. The place it splits is oil. What pulled June down was the drop in crude that came out of a temporary truce with Iran, and in July that truce broke and crude is rising again. If energy turns positive again in the July print, June was a one month affair. If core prices, with food and energy stripped out, keep coming down alongside it, the trend reading holds. The side that changed the yardstick and the side that asked for the change each had their own arithmetic. The Fed got a target that is easier to hit, and Congress got a way to shrink the deficit without losing votes. It is worth knowing how the number markets are cheering today was chosen.