Two shelves in the same aisle can respond to price three times apart, so a category average taken from outside your own shelf is worth very little.

Seven categories are now measured the same way, and the spread between them is larger than the spread inside any one of them. A discount on eye creams moves rank roughly one for one. The same discount on shampoo buys about four tenths of that. Both shelves sit in the same department, and both are stocked by the same kind of seller.

The second thing the seven have in common is that cuts and increases are not mirror images. In six of the seven, an increase costs less rank than an equal cut earns. That gap is where a price rise is affordable — but it is small enough in most categories that a wide range still covers both readings.

The third is price level. Within the categories that split cleanly into segments, the higher-priced group responds more strongly than the budget group, and it does so in both directions. Where a category shows no segment pattern at all, as face cleansers does, that is itself the finding: price works the same at €8 as at €40.