Cannibalization

A clear example: you discount one SKU, its sales rise 40%, but a similar SKU in the same book falls 15%. The customer did not switch brands; they just took the cheaper one.

To measure it correctly you must define the related product category in advance, measure sales of the whole category (not only the target product), and follow the same customers, because cannibalization happens in one customer’s basket, not in the market as a whole.