Schumer said the British company that developed the so-called footpath technology should use an opt-in mechanism to give shoppers the choice whether they want their movements tracked.
"The formula effect generally, and particularly in the case of clothing, is dependent not just on the choice of index but also on the characteristics of the price movements and levels being measured, sample design, choice of base period and price collection methods, " said its spokeswoman Jill Leyland.