Given incomplete factor markets, appropriate time paths of flow variables must be chosen to build required stocks of assets. That is, critical resources are accumulated rather than acquired in "strategic factor markets" (Barney [Barney, J. 1986. Strategic factor markets: Expectations, luck, and business strategy. Management Sci. (October) 1231--1241.]). Sustainability of a firm's asset position hinges on how easily assets can be substituted or imitated. Imitability is linked to the characteristics of the asset accumulation process: time compression diseconomies, asset mass efficiencies, inter-connectedness, asset erosion and causal ambiguity.competitive advantage, resource accumulation, imitation

The focus of this paper is on three major questions: (1) what is the theoretical rationale for the strategic group concept?; (2) does strategic group membership have performance implications?; and (3) are strategic groups and membership in strategic groups stable characteristics of industries? A statistical procedure is proposed to longitudinally identify strategic groups. The empirical setting is the U.S. pharmaceutical industry over the period 1963–82. While performance differences are found in terms of market share, profitability differences between groups are not observed. Neither are differences found in terms of risk and risk-adjusted performance. These results are attributed to the existence of performance variation within each strategic group. A dynamic model of competitive repositioning is proposed which helps integrate the findings from the study.

Whether and why members of the same strategic group would experience different performance results has received little attention in previous research. These questions are addressed in this paper. First, conventional theory on the relationship between firm performance and strategic group membership is reviewed. Then a theory is developed as to how historical differences among strategic group members may result in performance differences. An empirical analysis of risk and return relationships is conducted, centered on the nature of environmental change characterizing the industry. The empirical setting throughout is the U.S. pharmaceutical industry over the period 1963–82.

An analysis of the U.S. pharmaceutical industry during the period 1963–82 finds that a substantial decline in industry profitability is not explained by changes in the number and size distribution of firms, in segment interdependence and in strategic distance. In contrast, declining industry profitability is strongly associated with increasing rivalry. This increasing rivalry is associated with changes in strategic group structure and a concomitant shift from within group rivalry to between group rivalry.

Authors' reply to comments regarding their paper (Dierickx, I., K. Cool. 1989. Asset stock accumulation and the sustainability of competitive advantage. Management Sci.).