A Rotorua household watching their grocery receipts in late 2013 would have seen prices tracking steady, just like they had for years. The food price index sat at 15,317, barely a tick different from 2012 or 2011. Then something happened.

By 2014, that same index had fallen to 7,683. Not down a few percent. Down by half. In one year.

This isn't the story of Rotorua suddenly becoming New Zealand's cheapest place to eat. This is the story of a measurement change so dramatic it erased half the scale overnight, and nobody stopped to explain what it means for anyone trying to track what they're actually paying at the checkout (Source: Stats NZ, food-price-index-detailed).

Food price indices are supposed to be boring and reliable. They track the same basket of goods over time so you can see whether your grocery bill is climbing faster than your wages. When the number goes up, food's getting more expensive. When it goes down, you're catching a break. When it gets cut in half, something's broken.

The 2014 figure represents the lowest level Rotorua's food price index had recorded in 21 years. You'd have to go back to 1993 to find a comparable number. But nobody in Rotorua remembers 2014 as the year their grocery bills returned to early-90s prices. Because that's not what happened.

What likely happened: Stats NZ changed how it measures food prices. New base year. New methodology. New scale. The kind of technical recalibration that makes sense to statisticians and renders every historical comparison meaningless to everyone else.

Here's why that matters. When you're trying to understand whether your rent's rising faster than your food, or whether moving regions might ease your cost of living, you need data you can compare across time and place. A sudden halving of the index doesn't tell you food got cheaper. It tells you the ruler changed length.

For four years, Rotorua's index held steady around 15,000. Then it dropped to 7,683 and kept measuring from there. Anyone looking at that chart without context would think they'd missed the greatest grocery price collapse in modern history. Anyone trying to use it to make decisions about their household budget would be flying blind.

This is the invisible problem with official statistics. The measurements matter enormously when you're deciding whether you can afford to keep living where you live, or whether your wages are keeping pace with your bills. But when the methodology shifts and the numbers jump, you're left guessing which version of reality to trust.

Rotorua's food prices didn't halve in 2014. But the data did. And unless you know that break exists, you're comparing apples to something that isn't even fruit anymore.