Loyalty discounts and price-cost tests

We analyze, by means of a formal economic model, the use of the discount-attribution test to assess the competitive effects of loyalty discounts. (The discount-attribution test is a variant of the price-cost test, where the discount is attributed only to the share of total demand that is regarded as effectively contestable.) In the model, a dominant firm enjoys a competitive advantage over its rivals and uses market-share discounts to boost the demand for its own products. In this framework, we show that the attribution test is misleading or, at best, completely uninformative. Our results cast doubts on the applicability of price-cost tests to loyalty discount cases.