One item, many pack sizes

Search this site for tomatoes in Canada and you get a wall of rows with different numbers on them. They are not disagreeing with each other and none of them is wrong. Each one is the price of a different physical thing, and the difference is written in a field most people skim past: the pack description.

The pack description is part of the price's identity

Agriculture and Agri-Food Canada publishes its daily wholesale quotes with the trade's own pack notation attached. On the reading below, tomatoes alone appeared under twenty-nine distinct pack descriptions. These are the busiest of them.

Look at the three twenty-five pound cartons in the middle. Same weight, three separate series, because the trailing grade in the description distinguishes them and the source prices them separately. Averaging those three together would produce a number that describes no purchase anyone could make.

How much of this there is, and where

Pack fragmentation is a property of the provider, not of food. Some agencies publish per kilogram and are done with it. Others publish the trade's notation as they receive it.

The two extremes are worth naming. The Canadian feed carries more pack descriptions than it does item names, so the average item there spans several packs. The UK feed publishes most of its rows per kilogram, and the rest under a handful of counting units for things not sold by weight, so a UK item is usually one series and easy to follow.

Why nothing here is converted to a price per kilogram

The obvious wish is for the site to divide everything through to a common unit so the numbers line up. It does not, and the reason is in the table above rather than in anyone's preference: many of these descriptions contain no weight to divide by. A count of twenty-four in a carton is a count of twenty-four, and the source does not say what one of them weighs. Guessing a weight would put an invented figure into a series whose entire value is that it was not invented.

The comparison therefore runs on the provider's own string. Two observations are in the same series only when the country, provider, item name, economic level, market, unit and currency all match, and the unit in that list is the pack description exactly as published. That is what stops a movement being calculated between a five pound carton and a fifty pound bag.

Working with it

1. Find the pack description closest to what you actually receive, and treat that as your series. If you buy twenty-five pound cases of large round tomatoes, the extra-large series is a different product for this purpose.
2. Stay in it. The value of a series is the movement over time, and switching packs halfway through turns a price change into a packaging change.
3. Do not average across pack descriptions to get a headline number. If you need one figure for a category, take the percentage movements, which are comparable across packs, rather than the prices, which are not.
4. Check the market field before comparing two rows from the same country. Some providers name the market a quote came from and some leave the field empty, and the market is one of the seven fields the comparison matches on.
5. Read the unit off the card. Every card on the data explorer prints the currency and the pack description beside the value, so you never have to assume which series you are looking at.

None of this is friction for its own sake. It is the difference between a movement you can take to a supplier conversation and a movement that evaporates the moment someone asks which pack it was. If you are turning one of these series into a plate cost, the calculators take your own numbers and never store them, and the methodology page sets out the rest of the comparability rules.