The Centre for Science and Technology Studies (CWTS), Leiden University, has developed a new ranking system entirely based on its own bibliometric indicators. This web-publication is the first in a series of rankings. The work focuses on all universities worldwide with more than 700 Web of Science indexed publications per year. This implies that the about 1000 largest (in terms of number of publications) universities in the world are covered, and that our bibliometric analysis is based on the scientific output of many hundreds of active researchers in each of these universities.

The Leiden ranking benefits from all experiences and know-how available at CWTS: very accurate definition and 'unification' of universities worldwide; corrections for practically all errors and inconsistencies in the raw publication and citation data; thorough methodology based on 20 years of experience in research performance analysis; multiple-indicator approach. This latter point is very important: on the basis of the same data and the same technical and methodological starting points, different types of impact-indicators can be constructed, for instance one focusing entirely on impact, and another in which also scale (size of the institution) is taken in to account. Rankings based on these different indicators are not the same, although they originate from exactly the same data. Moreover, rankings are strongly influenced by the size-threshold used to define the set of universities for which the ranking is calculated.