Every year, 9,000 professors and 150,000 students at more than 300 universities are interviewed. The underlying criteria include the duration of studies, grades, and the personnel and technical facilities of the faculties. A distinction is made between research and teaching ranking.
The results show that very few universities obtained an excellent rating for all faculties, and that even within a subject, there may be large qualitative differences between teaching and research. When choosing a university, it is therefore advisable to take into account the quality of teaching in the respective subject.
You can find more detailed information about the ranking here.