Results from comparative international large-scale assessment studies, such as TIMSS and PISA generally reflect a downward trend for Swedish students’ results in subjects such as mathematics, science and reading comprehension. Descriptive analyses are compiled after each assessment, but there is a lack of more analytical analyses. Nevertheless, presented results receive a lot of attention in the public arena and educational investments are often based on results from these assessments. A valid question is on what groundwork the decisions are made, i.e. the interpretations and conclusions that reinforce the political decisions. It is fundamental that the assessment instruments (the subject tests and the background questionnaires) provide reliable and valid information about the concepts and abilities they intend to measure. It is also of vital importance that this works in a similar manner for all students in the examined country. Furthermore, if the international comparative studies should be able to make a substantial contribution to the knowledge of educational science field, there is an urge for deepened and extended analysis of data from these assessments. In addition, it is important to use the vast amount of collected background information (questionnaire data) in order to develop schools and thereby promote student success, not least in the light of the growing focus on assessments which imply that students and the teachers devote considerable time in participating in assessments and to preparations. The overall purpose is to examine the quality of the information TIMSS and PISA provide for different groups of students and to examine the relevance of this information for development of the Swedish school and thereby enhance student success. This is conducted by content analysis of the instruments and statistical analyses and modelling of the students’ test achievements. In order to take into account the hierarchical data structure of TIMSS and PISA and control for the student backgrounds when analyzing the results, multilevel analysis will be used. The results will contribute to unique knowledge of how Swedish student performance on large-scales assessment, relate to their national test results, grades and home background. The project will also shed light on how TIMSS and PISA relate to each other and how we can use data from TIMSS and PISA to develop the Swedish schools and thereby enhance student success. An important aspect will be to put the Swedish students’ results in an international context through country comparisons. The results will be published in international peer-review journals and presented on international conferences.