What is the relevance, in order to assess vocational skills, of observing the evaluated subject in a work situation, and then of having a discussion about their activity in that situation? The subject of this paper adheres to the theoretical framework proposed for vocational didactics. It also questions the interest of a vocational skill assessment methodology which, beyond the observation of a performance in a given situation, tries to forecast the “repeatability” of this performance by investigating afterwards the reasoning applied in this situation. This is the framework in which an experiment about the link between performance and conceptualization has been conducted. This paper presents the results of this experiment. Our research has been realized in the field of the professional use of a word processing software, but the conclusions go far beyond this particular context.