This article gives a critique of the methodology of OECD's Programme for International Student Assessment (PISA). It is argued that PISA is invalidated by the fact that the methodology chosen does not constitute an adequate operationalisation of the question of inquiry. Therefore, contrary to the claims of PISA, PISA is not an assessment of the ‘knowledge and skills for life’ of students, but only of ‘knowledge and skills in assessment situations’. Even this latter form of assessment is not fully reliable, however, because of problems at the level of concrete test items and because of an inherent confusion of relative and absolute evaluation.