Sorry, but he has a perfectly valid point. Defining the relevant market is The problem in antitrust / competition policy. He deducted that from the formula. Tha

Sorry, but he has a perfectly valid point. Defining the relevant market is The problem in antitrust / competition policy. He deducted that from the formula. That's astute. If you fail to define the relevant market, you fail to spot the failing market. This goes both ways. Sometimes products that are not even that similar work as substitutes preventing the exercise of market power (say red bull and coke). Other times market power in one narrow product group can work as a stepping stone for extending market power over various markets (say, the way a firm like Proctor and Gamble operates).