PISA has an extensive impact on policymakers, academics and the wider education community – but are the results valid? The overall aim is to investigate the validity of the test scores of PISA 2018 in Sweden. To reach that aim, we will investigate if the PISA sample is biased and determine whether students’ test scores translate into their national test results and grades. There is a gap in knowledge regarding the relation between students’ performances in PISA and their performances on high-stake tests. However, in this study we can link PISA with register data from Statistics Sweden (SCB). First, we will determine whether and how the exclusion rate and missing number of students affected Sweden’s overall PISA score. Second, we will define the relationship between PISA scores, grades and national test-results for the students who took part in PISA 2018. Third, we will investigate to what extent students’ test motivation in PISA explains discrepancies between PISA and the national assessments, and if test motivation varies for different student groups. In the short-term, we provide new information on how students’ results in PISA relate to their performances on high-stakes tests. If we find questionable validity of PISA-results we suggest to either correct PISA scores for test motivation or to incentivize students to take the test more seriously. In the long-term, our results could motivate changes regarding the way PISA reports results to students, schools, and the public.