As New Zealand's first national infrastructure plan promises billions in spending, there's a number buried in Stats NZ data that looks like either a miracle or a catastrophe: Timaru's food price index dropped from 15,445 in 2013 to just 7,836 in 2014.

That's not a typo. On paper, food costs in Timaru appear to have been cut in half overnight.

Except they weren't. What actually happened is more instructive than any price drop — it's a reminder that official statistics can shift beneath your feet in ways that make year-on-year comparisons meaningless.

The most likely explanation: Stats NZ changed its methodology. Food price indices get rebased periodically — the basket of goods gets updated, the reference year shifts, weights get recalculated. When that happens, the numbers before and after the change aren't comparable. You're measuring two different things with the same label.

This matters right now because everyone is watching food prices. When you're trying to figure out if your grocery bill is actually getting worse or if it just feels that way, you need data you can trust. But if the index can halve overnight due to a technical adjustment, how do you know what's real? (Source: Stats NZ, food-price-index-detailed)

The trajectory leading up to 2014 shows steady, predictable growth: 14,599 in 2010, climbing to 15,445 by 2013. Then the plunge. No economic event in Timaru — not a supermarket price war, not a farming boom, not a government intervention — could explain that drop.

Here's what this teaches us about reading official numbers: context is everything. A number without explanation is just noise. When food price data moves 49% in one year (the drop from 2013 to 2014), your first question shouldn't be "what happened to prices?" It should be "what happened to the measurement?"

This is particularly relevant as Kiwis watch their grocery bills climb month after month. Inflation since 2019 has been roughly 20-25%, which means even if a food price index stayed flat, you'd be paying a quarter more in real terms. Add in methodological changes and you've got numbers that look like they're telling one story while reality tells another.

The lesson: when someone quotes you a food price statistic, ask three questions. What year is the base? Has the methodology changed? And are these nominal dollars or inflation-adjusted?

Because in Timaru in 2014, the number said food got cheaper. But nobody's grocery bill agreed.