A measure of market share mobility is derived from an hypothesized model of underlying market share behavior. Explicit linkage of the two exposes some properties of the index and its implied behavioral assumptions and informs empirical work. Results for the U.K. newspaper industry offer some evidence of stable, overall mobility relationships, but the absence of long-run equilibrium relationships at the market share level suggests these may be misleading. Further work at the latter level seems likelier to advance understanding of both market share behavior and mobility, while care is required when supplementing concentration data with mobility statistics in antitrust cases.

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