Product market definition in antitrust cases when products are close substitutes or close complements.

I. Introduction

Judicial acceptance of the Department of Justice (DOJ) Merger Guidelines (1) over the past decade has standardized the methodology economists use to define markets in antitrust cases. This conceptual success, however, masks the fact that economic practitioners continue to reach divergent market definitions using identical methodology. This article will address two areas of continuing divergence: What can economic practitioners do to show that two products are close substitutes that belong in the same product market? and How should practitioners define product markets when complementary product relationships exist?

II. Product markets ...