Trimmed Mean vs Headline CPI

Key takeaways

• Trimmed mean removes the most extreme price changes at both tails each period.
• Headline can be pushed around by fuel, energy rebates and seasonal food.
• The RBA targets consumer price inflation of 2-3% on average over time.
• Headline above trimmed mean means volatile items are doing the work.

How is the trimmed mean calculated?

The ABS ranks every item in the CPI basket by its price change for the period, then removes a fixed share of the weighted distribution from each tail - the largest increases and the largest decreases - and averages what remains.

The point is not to exclude particular categories permanently. An item that is volatile this quarter may be well behaved next quarter, and the trim adapts. That is what distinguishes it from a simple "ex food and energy" measure, which excludes the same categories regardless of whether they were the problem.

The ABS also publishes a weighted median, which takes the price change at the middle of the weighted distribution. Both are underlying measures and they usually tell a similar story.

When do headline and underlying diverge?

Whenever something large and temporary hits the basket. The gap between the two is the signal.

Underlying inflation rising is the harder problem, because it usually means price pressure is broad rather than concentrated - and broad pressure is what monetary policy is actually for.

Monthly or quarterly - which matters more?

The ABS publishes a monthly CPI indicator as well as the quarterly CPI. The monthly series arrives faster but historically covered a smaller share of the basket in any given month, which made it noisier, and its trimmed mean is correspondingly less reliable than the quarterly equivalent.

The quarterly release remains the more complete measure, and the underlying series within it is the one that carries the most weight in policy discussion. A single monthly print that surprises is rarely enough on its own to change a rate decision - which is why markets often move on a monthly figure and then partially reverse when the quarterly arrives.

Headline inflation peaked at 4.6% over the year to March 2026 and has eased every month since - 4.2% to April, 4.0% to May, 3.8% to June and 3.5% to July - while the trimmed mean has been stickier, holding at 3.6% to July. Both remain above the 2-3% target band. That gap between a falling headline and a flat underlying measure is exactly what matters for borrowers, because policy responds to the underlying series.

Frequently asked questions

What is trimmed mean inflation?

A measure of underlying inflation that removes the largest price increases and decreases from the CPI basket each period and averages the rest. It filters out volatile one-off moves so the remaining figure better reflects persistent price pressure.

Why does the RBA prefer underlying inflation?

Because monetary policy cannot influence a global oil price or a one-off excise change, and reacting to temporary shocks would make policy more volatile without improving outcomes. Underlying measures isolate the persistent component policy can affect.

Which is higher, headline or trimmed mean?

Either can be higher and the relationship changes over time. Headline above trimmed mean means volatile items are pushing prices up; headline below means something temporary is holding measured inflation down, such as an energy rebate.

What is the RBA's inflation target?

Consumer price inflation of 2-3% on average over time. The target is expressed as an average rather than a point, which gives the RBA room to look through temporary deviations.

Related reading

• How the RBA Actually Sets the Cash Rate: The Reaction Function
• The 2026 Rate Reversal: How Three Hikes Undid a Year of Cuts
• NAIRU: Why Unemployment Drives Your Mortgage

Sources

• Consumer Price Index, Australia — Australian Bureau of Statistics
• Monthly CPI Indicator — Australian Bureau of Statistics
• Statement on Monetary Policy — Reserve Bank of Australia