Picture a Timaru household doing their weekly shop in December 2013. They're paying prices tracked at an index level of 15,445. Twelve months later, that same basket of goods registers at 7,836. The number has been cut nearly in half.

This isn't a typo. (Source: Stats NZ, food-price-index-detailed) The food price index for Timaru collapsed by 49.3% between 2013 and 2014. To find a comparable figure, you have to rewind to 1993.

But here's what that number actually tells us: something broke in how this data was collected or categorised. Food prices don't halve in a year. Not in Timaru. Not anywhere.

The trajectory makes this clear. From 2010 to 2013, Timaru's index moved like you'd expect: 14,599 in 2010, creeping up to 15,309 in 2011, 15,343 in 2012, 15,445 in 2013. Steady, predictable increases. Then the floor falls out.

What likely happened: a methodological change, a reclassification, or a shift in what Stats NZ was measuring. Maybe the sample size changed. Maybe the basket of goods was recalibrated. Maybe Timaru was grouped differently with other regions.

This matters because we trust these numbers to tell us what's happening to household budgets. When an index halves overnight, it doesn't mean Timaru families suddenly had twice the purchasing power. It means the data itself changed shape.

And that creates a problem. If you're comparing 2014 to earlier years, you're not comparing apples to apples anymore. You're comparing two different measurement systems. Any trend line that crosses 2014 becomes unreliable.

Stats NZ publishes these indices to track real changes in what Kiwis pay for food. But when the measurement shifts, the story blurs. Was 2014 genuinely the cheapest year for Timaru groceries in two decades? Or did we just change what we were counting?

The answer matters. Because if policymakers, researchers, or journalists use this data without noticing the break, they'll draw conclusions from a fiction. They'll compare 2015 to 2010 and miss that the ruler changed length halfway through.

The lesson: data doesn't lie, but it can tell you a story that isn't true if you don't know where the breaks are. Timaru's 2014 collapse isn't about cheaper bread or milk. It's about what happens when the numbers stop measuring the same thing.