Summer of stats part 2 – Of lettuces, strawberries and things

By Guest Author 27/12/2009

A running series from Statistics New Zealand helping us make sense of the Food Price Index and the Consumers Price Index…


Statisticians are not particularly noted for their vegetarian inclinations, but they do exhibit an appetite for fruit and vegetables. They find lettuces especially fascinating.

Source: Statistics New Zealand
Source: Statistics New Zealand

The Food Price Index works on the basis of price per kilogram. This is simple enough for oranges and apples because retailers sell them by the kilogram. But when items are priced as so many dollars ’each’, Statistics NZ has to calculate a per-kilo price. So the food price collectors weigh pineapples and heads of broccoli, cabbage, cauliflower …and lettuce.

In season, we tend to expect vegetables to be plentiful and of good quality. This is true for lettuces across our warmer months — from October to March they have weighty heads. And they are also at their cheapest. In fact, the bigger the head, the less you are likely to pay for a lettuce.

Conversely, when lettuces are smaller, you’d expect the price to be lower. But no. The smaller the head, the more you pay for it. It seems that a double whammy applies to lettuces — as their heads shrink, their price expands.

Statisticians also observe that there seems to be more out-of-season fruit in our shops these days. They know this because of statistical ’non-use’. If collectors can’t find enough of any one item on their weekly visits to retailers, that item can’t be included in the month’s Food Price Index. The price is not used. Non-use gives Statistics NZ gets a good picture of what’s on offer, or not.

For example, nectarines used to be off the list for five months — now it’s only two months. Strawberries used to be unavailable between March and August — but there are now plenty on the shelves in every month apart from May and June.

So if you follow the expert chefs’ advice and base your meals on fresh produce, it’s becoming more likely you can serve almost anything — even lettuces.

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