10 Using contextual information in decision making P. Brezillon UP6, Case 169 University of Paris 6 4, Place Jussieu, 75252 Paris Cedex 05, France Tel : +33 1 44274721, Fax : +33 1 44277000 E-mail: patrick.brezillon@lip6fr J.-Ch. Pomerol UP6, Case 169 University of Paris 6 4 Place Jussieu, 75252 Paris Cedex 05, France Tel:+33 1 44 277008, Fax: +33 1 44 277000 E-mail: Jean-Charles .Pomerol@lip6fr Abstract Look-ahead reasoning leads to unmanageable decision trees due to the large number of possible actions and events. However, in decision practice, people reduce the complexity of the tree by using as much as contextual information as they can. In this paper, we explain and model the different ways of using contextual knowledge to reduce decision-tree complexity. From a theoretical viewpoint, we introduce first the notion of action robustness and action postponing. As a consequence, the reasoning moves from look-ahead to diagnosis and relies on macro-actions. Then, we illustrate how context can be modeled and used to simplify decision trees according to the notions introduced in the frrst part. Keywords Decision support system, context, action postponing, robust action, macro-action 1 INTRODUCTION We have claimed elsewhere (Pomerol and Brezillon, 1997) that multicriteria DSSs offer an interesting frameworlc for studying the reasons that prevent decision makers from practically using DSSs, whether these DSSs include knowledge-based modules (see also Brezillon and Pomerol, 1996, for a survey on this topic in the framework of knowledge-based systems) or not. Some of the main weaknesses Context Sensitive Decision Support Systems D. Berkeley, G. Widmeyer, P. Brezillion & V. Rajkovic (Eds.) © 1998 IFIP. Published by Chapman & Hall