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 paper deals with the design, the making and the evaluation of an interface in Computer Based Learning Environments. In it, we present the method adopted to set up software which proposes tasks to the students and collects data for the diagnosis module. We consider here the problems linked to the re-use of didactic know-how based on pencil and paper tasks and in particular how to transfer these tasks to a data processing environment. These problems are considered from the interface designer’s point of view. In an attempt to make this clear, we make a distinction between ergonomic problems connected with the functioning of the interface and problems related to the field of application.