Prices & inflation

How to read consumer price indexes (US, UK, NZ)

Three CPI series share a name and none of the numbers: bases, frequencies and levels differ. How to compare CPI-U, UK CPI and the Stats NZ index correctly.

"Consumer price index" sounds like one number, but the three indexes behind our Price Index desk — the US CPI-U, the UK CPI and the New Zealand CPI — differ in base year, frequency and level, so their values are not comparable even when their directions are. This note explains what each series measures and how to read them side by side without mixing them up.

All three indexes do the same conceptual job: measure the price of a fixed representative basket of goods and services relative to a reference period. But each is produced by a different agency (the U.S. Bureau of Labor Statistics, the UK Office for National Statistics, Stats NZ), with its own basket definition, reference period, publication calendar, and revision practice. Younivi republishes the official series as-is and labels the one figure we compute ourselves — the year-over-year change — as derived.

The three series at a glance

Levels are not comparisons; growth rates are

The most common misuse of CPI data is comparing levels across countries. A US level of 334.980 against a UK level of 142.9 does not say anything about which economy's prices are higher — the bases are different (1982–84 versus 2015) and the baskets differ too. Each index says "prices in the basket cost X times what they cost in the reference period, in this country, for this definition of the basket." Change the reference period and the level changes; change the basket and it changes again.

Growth rates — the year-over-year percentage change — are meaningful within a series and roughly comparable across them, because ratios cancel the base out. Our desk computes each year-over-year change from the published levels: for the US, 334.980 against 323.976 a year earlier is the derived +3.4%; for the UK, 142.9 against 139.0 is +2.81%; for New Zealand, 1359 against 1306 is +4.06%. The "derived" label matters: it is Younivi's arithmetic on official levels, not each agency's headline inflation figure, which may use seasonally adjusted or rounded inputs.

Frequency makes behaviour look different

The US and UK series are monthly; the New Zealand series is quarterly. This changes texture, not truth. A quarterly index updates four times a year, so it looks smoother and reacts later to the same underlying movement; a monthly series shows every wiggle of energy prices and seasonal patterns. Comparing a Q2 quarterly print with a July monthly print — as this snapshot does — pairs observations from different dates and is fine only if you note the dates.

Frequency also affects revisions in practice. Monthly series are often revised or supplemented in the months after release; a quarterly index bakes more months into each observation. Neither practice is wrong; they are different publishing conventions, and readers who need the agency's own adjusted series should follow the citation on the desk page.

Not seasonally adjusted, and why that matters

The US series Younivi carries is the not-seasonally-adjusted CPI-U. That choice is deliberate: it is the unmodified official observation. Unadjusted indexes include recurring seasonal patterns — prices that rise predictably at the same time each year — so a first-time reader should not interpret every month-on-month movement as "inflation." Year-over-year comparisons largely cancel the seasonal pattern, which is why they are the more robust way to read an unadjusted series over longer spans.

Observation counts and history depth

The published history differs enormously: 1,364 monthly observations in the US since 1913's compilation; 463 in the UK on the 2015 base; 422 quarterly in New Zealand. Depth matters for questions like "how often has inflation run this fast?" — answerable in the US series, barely answerable in shorter series. It also changes how an index number looks: the US level (334.98) is large mostly because its base is forty years old; the UK level (142.9) is small because its base is recent. Neither implies higher or lower inflation.

When the producer changes the index

All three agencies periodically rebase their CPI — change the reference period, update the basket, or revise methodology — and splice the history onto the new base. After a rebasing, the printed levels change even though past inflation rates do not. Younivi republishes whatever the current official series is; a chart on our desk can therefore shift level if the source rebases, and our methodology notes flag that the source, not Younivi, made the change.

Where this fits in the wider picture

A recurring reader reaction to any consumer price index is that measured inflation feels lower than lived inflation. The gap is usually methodological rather than a matter of honesty. An index is a fixed representative basket averaged across households, while any one household buys a different mix: the index weights housing, food, energy and services by population shares, but an individual's experience is weighted by their own spending. When the categories a household buys most are the ones rising fastest, that household's personal inflation runs above the published average, and the reverse is equally possible.

Owner-occupied housing is the best-known example of why countries are not directly comparable. Some statistical systems put a rental-equivalence charge for owner-occupiers inside the index; others handle housing through separate measures or leave it out of the headline basket altogether. The United States, the United Kingdom and New Zealand each make different choices here, so the same underlying housing pressure can show up differently in each published figure.

This is why the index should be read as a consistent yardstick rather than a description of any one budget. Its value is that the same basket is priced the same way over time within a country, and harmonised as far as possible across countries, so that changes and comparisons mean something. A reader who wants their own cost of living is asking a different question from the one the index answers.

How to read this on Younivi

The CPI pages on the Price Index desk list each series with the producing agency, base period, frequency, latest observation and the year-over-year change (labelled where derived). Companion notes cover the New Zealand house-price index and other indexed series. Figures are republished official data, may lag the producer, and are not financial advice.