the overcharge has a one-to-one relationship to the lerner index, the most common measure of market power in economics. both measures employ the same numerator (pm - pc), where pm is the observed market price and pc is the competitive benchmark price, but use different denominators. the overcharge ratio is (pm-pc)/pc, whereas the lerner index is (pm-pc)/pm. both ratios are zero when a market is perfectly competitive or a cartel is ineffective in raising prices. the lerner index has an upper limit of one when the pure monopoly price is charged in a market. the overcharge has no upper limit.