In 2010, Timaru's annual food price index reading sat at 14,599. By 2013, it had crept up to 15,445. Then, in 2014, it plummeted to 7,836.

That's not a typo. It's not a correction. It's the kind of number that makes you stop and ask: what on earth happened in Timaru?

Here's what didn't happen: Timaru's supermarkets didn't suddenly slash prices in half. Food didn't become 49% cheaper overnight. Residents didn't start bulk-buying in Ashburton to avoid local shops.

What did happen was something far more mundane, and far more important for anyone trying to understand economic data: Stats NZ changed how it measures food prices.

Between 2010 and 2013, the numbers climbed steadily. Each year added another hundred or so points to the index. The trajectory was clear, predictable, boring. Then the methodology shifted, the basket of goods was reweighted, or the base year was reset. The result? A figure that looks like a freefall but is actually a recalibration.

This is the problem with index numbers. They're designed to track relative change over time, not absolute levels. When the measurement system changes, the numbers before and after the break aren't directly comparable. You can't draw a line through 2010 to 2014 and call it a trend. The line breaks in the middle.

What makes this particularly frustrating is that 2014 now sits as the lowest reading in 21 years. Go back to 1993 and you'll find a similar figure. But that comparison is meaningless if the two numbers weren't calculated the same way.

For Timaru residents, none of this changed what they paid at the checkout. The price of bread, milk, and mince didn't care about index methodology. But for anyone trying to track cost-of-living pressures over time, trying to compare regions, or trying to understand whether things are getting better or worse, this data break matters.

It's a reminder that numbers don't speak for themselves. They need context, continuity, and a clear explanation of what changed when the trend line suddenly nosedives (Source: Stats NZ, food-price-index-detailed).

The real story here isn't about Timaru's food prices. It's about what happens when data systems evolve faster than the public's ability to interpret them. It's about the gap between what a number appears to say and what it actually means.

So the next time you see a dramatic swing in an economic indicator, don't assume the economy moved. Ask whether the goalposts did.