Abstract
In this paper we present a qualitative study on data collected by an eye-tracker tool regarding a Calculus task. One purpose of this research is to highlight the differences and similarities between visual observation of expert and non-expert groups. Analysis of the way of reading a text can provide a lot of information about cognitive processes carried out to solve the task. Moreover, the aim of this study is to analyse, through the eye-tracker tool, the difficulties of students concerning the concept of derivatives and to understand what may trigger a wrong answer to the task.