A Rotorua household doing their weekly shop in 2014 saw something strange in the data: the region's food price index had been sitting comfortably above 15,000 for years, then suddenly dropped to 7,683. That's not a correction. That's a cliff.

To find a number that low for Rotorua, you have to go back to 1993. For three consecutive years leading up to 2014, the index hovered around 15,300. Then it halved. (Source: Stats NZ, food-price-index-detailed)

This isn't a story about Rotorua families suddenly paying half as much for bread and milk. Food prices don't work that way. What likely changed was the methodology, the basket of goods being measured, or how regional data was being collected and reported. But here's the problem: when the index changes this dramatically, it breaks the one thing these numbers are supposed to provide, which is comparability over time.

Food price indices exist so households can track whether their grocery bills are getting more or less affordable relative to the past. A single-year drop of 50% in the index makes every comparison before and after 2014 unreliable for Rotorua residents trying to understand their own cost of living.

The timing matters. By 2014, food inflation had become a major political issue across New Zealand. Families were feeling the pinch. Regional data was meant to show which parts of the country were hit hardest. But if Rotorua's index suddenly resets to half its previous value, how do you compare it to Auckland's, which stayed consistent? How do you track whether things got better or worse?

Stats NZ doesn't publish explainers for every regional quirk in its datasets. The numbers are there, but the context often isn't. For Rotorua, the 2014 figure stands as an outlier in a 21-year dataset, a number so dramatically different from what came before that it demands explanation.

This is the hidden cost of changing how you measure things: you lose the ability to tell a coherent story. The 2014 drop might be technically correct under a new methodology, but it renders two decades of prior data incompatible. For anyone trying to understand how food costs in Rotorua have changed since the 1990s, the answer is now: we don't really know.

The index has been recalculated or rebased before, and it will be again. But when it happens without a clear public explanation, it turns useful data into a puzzle that only specialists can solve. Everyone else is left wondering why the number that's supposed to reflect their lived experience suddenly doesn't.