The work presented here is part of a multidisciplinary project, called the PÉPITE project. Its aim is to set up a system to diagnose student profiles starting from a multidimensional analysis grid to evaluate their competences in elementary algebra. This article deals with the conception, the making and the evaluation of an interface in Computer Based Learning Environments. In it, we present the method adopted to set up a software which proposes tasks to the students and collects data for the diagnosis module. We consider here the problems linked to using didactic know-how related to pencil & paper tasks and in particular how to transfer these tasks to a data processing system. These problems are considered from the interface conceptor’s point of view. In an attempt to make this clear, we make a distinction between ergonomy problems connected with the functioning of the interface and problems connected to the field of application.