The focus of this research is the concept of median, which has received scarce interest in previous research. We analyse the open responses given by 518 Mexican students from Educación Secundaria (Junior) and Bachillerato (Senior) Secondary Education to a problem involving the computation of median. Using some ideas from the onto-semiotic approach, we classify the responses, taking into account the central tendency measure used, and describe the students' semiotic conflicts. We use the chi-square test to study possible dependence between responses and students' group. We observe better results in computation in Educación Secundaria students but better competence to select the best representative value in Bachillerato students.