Assessments are increasingly being designed on a digital artefact (computer or tablet), while in France the equipment rate in primary schools is still low. Some computer-based assessment tasks use specific software functionalities (the dynamic aspect of a geometric figure for example), while others "migrate" from paper and pencil (PP) to digital artefact without using specific functionalities of dedicated software. In this research, we are interested in the validity of assessment tasks designed on tablet (especially when students do not usually use a tablet in the classroom and/or in assessment situations) and in the effects of migration (from PP to tablet) on student performance and procedures from one medium to another.