In this work we study and experiment the integration of usage data in the design process of an interactive information system (IIS), specially in the scope of an IIS dedicated to decision-aid. Data have been generated by using methods of activity analysis. We make the assumption that for integrating usage data successfully, they must be represented in a formalism close to the development language. First a generic scheme of information flow in a IIS is defined. Each components of this scheme is then characterized. Data representing user's needs are then collected, analyzed and represented in an adapted knowledge representation scheme. We build a formal model of the knowledge, which has been collected, and then define a users needs matching function for linking the usage activity to the resources the IIS. For each identified problem corresponding to a specific users needs, the function brings out the building of a solution. The approach has been tested by a functional specification design tool developed for the specification of a traveller information system for aiding users of a public transportation network in the choice of the best transportation mode according to the network functioning conditions. It is built in three steps: 1) the construction of a method for collecting users needs data in different use cases; 2) the object-oriented modelling of corresponding knowledge and its transcription into the UML formalism; 3) the validation of the approach by implementing a tool for functional specification aid.