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Evolutionary theories of economic development stress the role of variety as both a determinant and a result of growth. In this paper we develop a measure of variety, based on Weitzman's maximum likelihood procedure. This measure is based on the distance between products, and indicates the degree of differentiation of a product population. We propose a generic method, which permits to regroup the products with very similar characteristics values before choosing randomly the product models to be used to calculate Weitzman's measure. We apply the variety measure to process characteristics of oil refining. The results obtained for this technology show classic evolutionary specialization patterns that can be understood on the basis of niche theory. Here the changes in variety are related to changes in the range of the services the technology considered can deliver, range which plays a role similar to that of the size of the habitat of a biological species.

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[fre] Bertrand Bellon, Jorge Niosi, Paolo Saviotti, Michael Crow Les systèmes nationaux d'innovation : à la recherche d'un concept utilisable. Cet article vise à établir les fondements du concept de système national d'innovation. Le développement économique actuel est marqué par la contradiction entre, d'un côté l'ouverture des frontières nationales entraînant l'égalisation relative des règles de la concurrence ; de l'autre, la différenciation croissante, entre espaces économiques, des stratégies et des résultats qui en découlent. Les comportements des acteurs sont, en effet, d'abord orientées vers la valorisation des spécificités existantes, bien au-delà de l'alignement sur des standards internationaux. La dimension technologique, ou plutôt innovative, redevient une des sources des avantages construits. Le concept de système national d'innovation participe aux théories évolutionnistes, ou plus précisément aux approches cybernétiques, au sens des sciences de la régulation et de la communication entre les hommes ou entre l'homme et la machine. Cet article cherche non seulement à fonder le concept sur le plan théorique, mais propose une méthodologie d'analyse concrète des différents systèmes nationaux d'innovation. [eng] Bertrand Bellon, Jorge Niosi, Paolo Saviotti, Michael Crow National system of innovation: an attempt to determine a concept. This article tends to develop the national system of innovation (NIS) concept. The current worldwide economic game is two folded. On one side, frontiers are increasingly open, which spread out more eavenly among countries the laws of competitiveness. On the other side, economic efficiency heavily depends on the specificities of each territory. Strategies are fully orientated towards the valorization of such specificities. Technology is back as a corner stone of built advantages among nations. The NSI concept is part of the evolutionnist theories. More precisely it belongs to the cybernetic approach of relations between men and between man and the machine. This article does not limit its attempt to the funding of a concept; it proposes an applied methodology for the analysis of specific NSIs.