Salt & spices

Each product contributes its own median across the chains that stock it, and the category median is taken over those — so a product carried everywhere does not count more than one carried by two chains. Prices are compared per kilo or per litre, never per pack, and a category is measured on the unit most of its products share.

The trend is measured on matched products: for each pair of consecutive points we take the average price change across the products priced in both — an average of each product's own change, so a product that appears or disappears moves the sample without moving the index. Those changes are then chained together. This matters here because the early points are rebuilt from recorded price history and see far fewer products than a week we read live.

The figure is the change over the last twelve months, which is the window this data supports and the one national statistics quote. Earlier points are on the chart but not in the claim: most of our recorded price history is recent, so the further back a figure reaches, the less it is measuring.

Points before the index started are rebuilt from each chain's own recorded price history, so their product counts are thinner than a week we read live. They are drawn dotted.

A week with too few products or too few chains publishes no figure at all rather than a thin one, and is drawn as a gap.

Prices come from the chains' own published price lists and can be stale or wrong. The shelf is the authority.