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Among the large number of economic indicators able to measure the performance of firms and territories, this paper proposes to investigate and apply the technical devices resulting from the "shift-share" model. This approach allows us to assess the "regional effect," which is defined as the economic position of the territory relative to the overall economic dynamic. When estimated at the sectoral level, this regional effect measures the specific position and performance of any firm according to its location. This approach is tested to determine the causes of the employment changes in the French regions between 2000 and 2008. It confirms that the contingent nature of the mechanisms of agglomeration on the one hand, and the role of local demand on the other influence the localized behavior of firms. JEL codes: D24, R11, R12