The headlines say that PCE (personal consumption expenditure) inflation fell in January. This is a case of “Yes, but …”. In particular, it is true that the year-over-year PCE inflation rates fell from 2.6% (December 2024) to 2.5% (January 2025) in the case of overall PCE inflation, and from 2.86% to 2.64% for core PCE inflation (that is, excluding food and energy). Looks like the Fed is doing a great job. However, the annualized month-over-month inflation rates jumped from 3.56% to 3.98% (PCE), and from 2.52% to 3.47% (core PCE).

How can we reconcile these divergent patterns to the year-over-year and month-over-month inflation rates? As we have previously explained, the year-over-year inflation rate is roughly the average of the past 12 month-over-month inflation rates. For example, the year-over-year inflation rate for December 2024 is the average of the 12 month-over-month inflation rates in 2024. Similarly, the January 2025 year-over-year inflation rate is the average of the 12 month-over-month rates running from February 2024 through to January 2025. As a result, every month the calculation for the year-over-year inflation rate adds in to this average the current month reading of the month-over-month inflation rate while simultaneously dropping out the monthly rate from 13 months ago. With all of this in mind, for core PCE inflation, the year-over-year rate for January 2025 added in the January 2025 month-over-month rate (3.47%) while dropping the January 2024 rate (6.14%). In this case, the year-over-year rate for January 2025 fell not because of a particularly favorable month-over-month rate for January 2025, but because of an especialy high rate for January 2024!

It is for reasons like this that we developed our measure of trend inflation. Without getting into the details, our measure of trend inflation is a “constant gain” measure of the average month-over-month inflation rates: it applies a constant weight to the current month-over-month rate, with the remaining weight applied to the previous month’s reading of the trend. Our measure of trend PCE inflation rose from 2.52% (December 2024) to 3% (January 2025) while our measure of trend core PCE inflation rose from 2.4% to 2.76%.
Another desirable feature of our trend inflation measure is that it responds reasonably quickly to changes in underlying trend inflation. Visually, we can see that the month-over-month inflation rates have trended up over the last half year. Our measure of trend reflects what can be seen in the data. While the year-over-year inflation rates will eventually also reflect such a change in underlying trend, it will take nearly a year to fully reflect the change in trend. In 2021, inflation rose sharply and the Fed appeared to be asleep at the switch. In particular, our trend measure was rising pretty quickly during 2020 and moved over the 2% target by midyear while the year over year measure stayed below 2% throughout the year and into 2021. One can only hope that the Fed learned something from its earlier mistake.