Methodology

In short

The monthly Price Monitor has three parts:

• Inflation monitor — how each supermarket’s prices move over time.
• Branded basket — the price level of branded products per supermarket.
• Own-label basket — the price level of own-label products per supermarket.

In every ranking the cheapest shop sits at 100; the others show how many percent dearer they are on average.

The light bar shows the uncertainty margin. When margins overlap, the order is not firm.

What do we measure?

Inflation monitor

Tracks how each supermarket’s prices move over time. The starting month is set to index 100; a reading of 100.5 means prices are 0.5% higher than the starting month. Because we include promotional prices, a retailer’s promotion calendar can tint a month up or down. It is not an official ONS/CPI figure.

Branded basket

Compares the exact same brand, variant and size across supermarkets, converted to a per-litre or per-kilo basis. The cheapest shop sits at 100; the others show how many percent dearer their branded products are on average.

Own-label basket

Compares comparable own-label and unbranded products. These are not identical across shops, so this is a price-level indication, not a comparison of the exact same product.

How do we compare prices?

Branded

✅ Compared — same brand, variant and size: Heinz Tomato Ketchup 342g ↔ Heinz Tomato Ketchup 342g.

❌ Not compared:

• a branded product against an own-label one (different brand);
• a different variant or flavour of the same brand.

Own-label

Own-label products differ per shop — there is no barcode that is the same everywhere. So we only compare products that are similar enough to make a fair price comparison.

✅ Compared — same kind of product, comparable variant and quantity: Tesco semi-skimmed milk 1L ↔ Sainsbury’s semi-skimmed milk 1L.

❌ Not compared — too different to be fair:

• whole milk vs. semi-skimmed milk (different variant);
• butter vs. margarine (different product);
• a size gap too large to compare fairly;
• a product with no recognisable comparable own-label at other shops.

We only compare two shops on the products they both stock; what only one shop carries does not count in that comparison.

How to read the ranking

The cheapest shop sits at 100. A shop at +7.0% is on average 7% dearer than the cheapest — across the whole basket, not per individual product.

The light bar shows the uncertainty margin. When two shops’ margins overlap, they are too close to claim a firm order.

A shop at the top is not automatically the cheapest on every individual product — it is about the average price level within the basket.

We weight categories with fixed CBS-style weights — how much an average household spends on each category — not by how many products we happened to find.

How do we find and assess shrinkflation?

How do we find cases?

We track pack sizes in online supermarket data. We look for a smaller pack on the same product listing and for possible replacements: a product disappears and a smaller product from the same brand appears. We compare product names, pack sizes and when products disappear and appear, among other signals. A match does not prove that the products are the same variant.

Who checks what?

Possible cases are assessed by AI or a reviewer. Not every published case has been checked manually. Assessment considers the size reduction and, for replacements, whether the products belong together. When a reviewer publishes a replacement case, we require evidence of the old regular price and an equivalent replacement. The current regular price per kilo, litre or unit must then be more than 2% higher. This price check is not yet applied to every case.

What are the limitations?

Supermarket data can be wrong or incomplete; AI and reviewers can also make mistakes. For size changes on the same product listing, the displayed price-rise percentage still assumes the pack price stayed the same before and after the change. For replacements we use captured prices, where available. The percentage therefore does not always establish a rise at the current price. Offers can change the price temporarily. The date found is not necessarily when the smaller pack first appeared in the shop. This register is not comprehensive.

Spotted an error? Contact us through Corrections with the case link and evidence, such as a photo or receipt.

What are the limitations?

• Every ranking is a snapshot on the measurement date; next month the order may change.
• Own-label products are comparable, not identical — the own-label basket is a price-level indication.
• Not all supermarkets publish sufficiently complete data; shops with insufficient comparable data are included partially or not at all.
• Allsupers is independent and not sponsored, and not a supermarket itself.