On paper, Timaru just experienced the most dramatic grocery price collapse in New Zealand history. In 2013, the town's food price index sat at 15,445. Twelve months later: 7,836. A 49% drop in a single year.

If that were real, every Timaruvian would be celebrating. A family spending $250 a week on groceries would suddenly be paying $127. Restaurants would be giving food away. Cafés would be deserted because cooking at home became absurdly cheap.

None of that happened. Which means we're looking at something else entirely: a fundamental change in how this data is collected or calculated. The number didn't halve because food got cheaper. It halved because the measurement system changed.

This matters because food price data isn't just numbers on a spreadsheet. It's how Statistics NZ tracks inflation. It's how employers justify wage negotiations. It's how government calculates benefit increases. And when the methodology shifts mid-stream, it breaks every comparison we try to make.

Look at the trajectory: steady climbs from 2010 onwards. 14,599 in 2010. 15,309 in 2011. 15,343 in 2012. Then 15,445 in 2013. Those numbers tell a story of gradual price increases, the kind you'd expect during a period of moderate inflation.

Then the cliff. The 2014 figure is the lowest Timaru has recorded in 21 years. You have to go back to 1993 to find something comparable. But 1993's economy bears no resemblance to 2014's. The dollar was worth more. Wages were lower. The entire food supply chain was different.

What actually happened? Likely a rebasing of the index. Statistics agencies do this periodically: they reset the baseline year, adjust the basket of goods they're tracking, or change how they weight different categories. It's standard practice. But it makes historical comparisons useless unless you know exactly what changed and when.

The problem is that most people don't read the methodology notes. They just see the number. And when politicians or commentators point to food price data to make an argument, they rarely mention that the 2014 figure isn't comparable to 2013's.

This is the invisible infrastructure of economic data: the rebases, the methodology shifts, the quiet changes that make year-on-year comparisons misleading. We trust these numbers to tell us what's happening to our wallets. But sometimes, all they tell us is that someone changed how they count. (Source: Stats NZ, food-price-index-detailed)