Mobility statistics are direct measures of the intensity of competition; market-structure indicators, indirect measures. In this regard the most widely employed measure of market structure is the concentration ratio. Despite the fact that structural measures provide only proxies for the extent of competition, they are widely used by professional economic staff during the formulation and administration of competition policy. This paper investigates the appropriateness of this approach. It concludes that while concentration in Canada's manufacturing sector has remained constant, this masks considerable mobility. The implications of this result are discussed.