Many studies in the field of education focus on the validation of testing instruments, such as questionnaires or assessment instruments. Indeed, such studies are published each year in the journal Mesure et évaluation en éducation. These studies can be classified in two broad categories: on the one hand, those in which psychometric procedures (e.g. factor analysis or Item Response Theory model like the Rasch model) constitute, in and of themselves, the validation process; and on the other hand, those in which relying on an expert panel to develop the instrument is seemingly enough to ensure validity. This article aims to show the necessity of using an argument-based validation process with quantitative and qualitative evidence by proposing two models which can be combined to help formulate arguments supporting the assessment instrument validation. The idea is to address the fact that if we do not understand what we are trying to demonstrate, the arguments are likely to be of little use.