What a comparison in real dollars can and cannot tell you

Converting an old amount into today's money looks like the simplest operation in economics: take two index numbers, divide one by the other, multiply. Both of those index numbers are the output of decisions that shape what the answer means.

Key Takeaways

• The ABS states that "the objective of the CPI is to measure pure price change over time".
• Quality improvements are stripped out, so a better product at a higher price is not counted as inflation.
• The hedonic method values product characteristics statistically when models cannot be compared directly.
• The index weights are updated and the series chain linked, and the ABS notes indexes "should not be chained at intervals less than annual".
• The result describes a national basket, not any individual household's spending.

Pure price change is a constructed quantity

A price index is trying to answer a question that reality makes awkward: what would the same thing cost now? The awkwardness is that very little stays the same thing.

The ABS states the objective directly: "The objective of the CPI is to measure pure price change over time, so ideally identical goods and services should be priced from one period to the next." The word doing the work is identical, and it is a condition the world does not supply. Products are discontinued, models are replaced, and the replacement is usually different.

So when a superseded model is swapped for its successor, the statistician has to decide how much of the price difference is a genuine price movement and how much is the value of the improvement. If a replacement costs more and does more, treating the whole increase as inflation would overstate it.

That decision is quality adjustment, and it is applied everywhere products change. It is the reason a price index is a constructed measure rather than an observation.

The hedonic method, and where it struggles

Where models differ in several ways at once, the ABS can use a hedonic approach: a regression in which the prices of an array of different varieties of a product are explained by the characteristics those varieties possess. The estimated relationship is then used to value the difference between an old model and a new one.

The technique has known limits, and the ABS names them. It requires substantial data and expense. And it handles only characteristics that can be quantified, which leaves out things that genuinely matter to buyers, such as the handling characteristics of a car or a change in fashion.

That limitation is worth carrying into any real dollars comparison. Where a category has improved in ways a regression cannot capture, an inflation adjustment does not fully describe the change in what the money buys. Comparing the price of a car across decades is not comparing the same purchase, and the adjustment can only go so far toward pretending otherwise.

Why the series is joined rather than continuous

The second decision concerns the basket itself, which cannot be held fixed forever without becoming a description of a country that no longer exists.

The Australian approach is to update the weights more frequently and chain link the series together to form a longer time series. Chain linking joins two indexes that overlap in a period by rescaling one so its value equals the other's in that period, producing a single continuous history.

There is a constraint on how often that can be done. The ABS notes that indexes "should not be chained at intervals less than annual", because chaining in a fixed weight index can introduce bias where it coincides with seasonal or cyclical peaks and troughs in prices.

Two things follow for anyone reading a long historical comparison. The history is a chain of segments rather than one measurement, and each segment was built on a slightly different basket. And a comparison stretching across many decades is comparing baskets that share a method rather than contents, which is a real limit on what the number means even though the arithmetic is exact.

Re-referencing is not re-weighting

A related confusion is worth separating out, because it produces wrong answers that look plausible.

Re-referencing changes the period in which the index equals 100. It shifts the level of the published numbers without changing the relative movements between periods. Linking, by contrast, is the weight-updating mechanism described above.

The practical consequence is that index numbers from two different published series cannot simply be divided into one another. They are expressed against different reference periods, and a different conversion factor applies to each index. A comparison is only valid within one continuous series, which is why an inflation calculator uses a single consistent series rather than whatever figure appears in a given historical document.

What the answer is a statement about

All of the above is machinery for producing a number about a national aggregate. It is not machinery for producing a number about a person.

The index is weighted by what Australian households collectively spend, so it describes an average that no household actually is. The Reserve Bank's explainer states the limitation plainly: the CPI "does not take into account the differences in spending patterns between individual households", and its example is exact, that cars carry a weight of almost three per cent in the basket while not every household owns one.

So a real dollars comparison answers what money bought, in aggregate, in the proportions the country bought it. Where a household's spending is concentrated in categories that moved differently from the average, its own experience diverged, and the divergence is not an error in the index.

Where to look next

The inflation calculator performs the comparison across the published annual series, the inflation rate page carries the current published figure with its source, and consumer price index defines the measure itself.

Sources

• Australian Bureau of Statistics, Quality change and new products - pure price change, quality adjustment and the hedonic technique
• Australian Bureau of Statistics, Re-referencing and linking price indexes - chain linking, the annual minimum interval, and re-referencing
• Australian Bureau of Statistics, Frequently asked questions about the Consumer Price Index - the cost of living distinction and the weighting sources
• Reserve Bank of Australia, Inflation and its Measurement - individual households against the average basket

Not financial advice, and not a recommendation of any product or institution. Reviewed by the Tradergy editorial team, September 2026.