Universities
The CWTS Leiden Ranking Open Edition 2025 includes 2831 universities worldwide. These universities have been selected based on their number of publications in the period 2020–2023. As discussed below, a sophisticated methodology is employed to assign publications to universities.
Identification of universities
Identifying universities is challenging due to the lack of clear internationally accepted criteria that define universities. Typically, a university is characterized by a combination of education and research tasks in conjunction with a doctorate-granting authority. However, these characteristics do not mean that universities form a homogeneous group that can be directly compared internationally across all dimensions.
The Leiden Ranking Open Edition includes universities that have at least 1500 publications in the period 2020–2023 indexed in the OpenAlex database. These publications may appear published in international scientific journals but can also be disseminated through outlets aimed at regional or local audiences that are more difficult for indexing services to identify and capture. For the Leiden Ranking Open Edition, it is therefore essential to interpret performance scores in the context of each institution’s mission and responsibilities, which are closely tied to national and regional academic systems. Such systems – and the role of universities within them – vary significantly between countries and continue to evolve. Inevitably, the results presented in the Leiden Ranking Open Edition reflect these differences and developments. In addition, it is important to consider that OpenAlex’s coverage of local literature varies substantially across countries, disciplines, and regions. The international variety in the organization of academic systems also poses difficulties in terms of identifying the proper unit of analysis. In many countries, there are collegiate universities, university systems, or federal universities. Instead of applying formal criteria, whenever possible we follow common practice based on the way these institutions are perceived locally. Consequently, we treat the University of Cambridge and the University of Oxford as entities, whereas in the case of the University of London we distinguish between the constituent colleges. For the United States, university systems (e.g. the University of California) are split up into separate universities. The higher education sector in France, like in many other countries, has gone through several reorganizations in recent years. The Leiden Ranking Open Edition 2025 includes French organisations that are designated as "établissements publics expérimentaux (EPE)". This is a type of HEI in France created by the law of 12 December 2018 in which different research and higher education institutions work together to eventually form a single HEI. Research and educational organisations that are part of a EPE as "établissements-composantes" will no longer be included as separate organisations in the Leiden Ranking Open Edition 2025.
Publications are assigned to universities based on their recent configuration.
Research Organization Registry (ROR) and affiliated institutions
A key challenge in the compilation of a university ranking is the handling of publications originating from research institutes and hospitals affiliated with universities. Among academic systems, a wide variety exists in the types of relations maintained by universities with these affiliated institutions. Usually, these relationships are shaped by local regulations and practices affecting the comparability of universities on a global scale. As there is no easy solution for this issue, it is important that producers of university rankings employ a transparent methodology in their treatment of affiliated institutions.
For the CWTS Leiden Ranking Open Edition we use the relationships between universities and their affiliated institutions included in the Research Organization Registry (ROR). For the classification of these relationships we use the same the methodology that is also used in the traditional Leiden Ranking.
CWTS distinguishes three different types of affiliated institutions:
- Component
- Joint research facility or organization
- Associated organization
In the case of a component, the affiliated institution is actually part of or controlled by the university. Universitaire Ziekenhuizen Leuven is an example of a component, since it is part of the legal entity of Katholieke Universiteit Leuven.
A joint research facility or organization is identical to a component except that it is administered by more than one organization. The Brighton & Sussex Medical School (the joint medical faculty of the University of Brighton and the University of Sussex) and Charité - Universitätsmedizin Berlin (the medical school of both the Humboldt University and the Freie Universität Berlin) are examples of this type of affiliated institution.
The third type of affiliated institution is the associated organization, which is more loosely connected to a university. This organization is an autonomous institution that collaborates with one or more universities based on a joint purpose but at the same time has separate missions and tasks. In many countries, hospitals that operate as teaching or university hospitals fall into this category. The Massachusetts General Hospital, one of the teaching hospitals of the Harvard Medical School, is an example of an associated organization.
A publication is counted as output of a university if at least one of the affiliations in the publication explicitly mentions either the university or one of its components or joint research facilities. In a limited number of cases, affiliations with institutions that are not controlled or owned by the university are also treated as if they were mentioning the university itself. The rationale for this is that in some cases institutions – although formally being distinct legal entities – are so tightly integrated with the university that they are commonly perceived as being a component or extension of that university. Examples of this situation include the university medical centers in the Netherlands and some of the academic health science systems in the United States and other countries. In these cases, universities have actually delegated their medical research and teaching activities to the academic hospitals and universities may even no longer act as the formal employer of the medical researchers involved. In other cases, tight integration between a university and an academic hospital may manifest itself by an extensive overlap in staff. In this situation, researchers may not always mention explicitly their affiliation with the university. An example of this tight integration is the relation between the University Hospital Zurich and the University of Zurich.
Our approach is discussed in more detail in this paper on academic hospitals.
Affiliated institutions that are not classified as a component or a joint research facility or treated as such are labeled as associated institutions. In the case of publications with affiliations from associated organizations, a distinction is made between publications from associated organizations that also mention the university and publications from associated organizations that do not include a university affiliation. In the latter case, a publication is not considered to originate from the university. On the other hand, if a publication includes an affiliation from a particular university as well as an affiliation from an associated organization, both affiliations are considered to represent that particular university. The effect of this procedure depends on the counting method that is used in the calculation of bibliometric indicators. The procedure influences results obtained using the fractional counting method, but it has no effect on results obtained using the full counting method.
Selection of universities
The Leiden Ranking Open Edition 2025 includes 2831 universities from 120 countries. These are all universities worldwide that have at least 1500 publications in the period 2020–2023 indexed in the OpenAlex database. A publication is taken into account only if it meets the following criteria:
- The publication is classified as article, review, or book chapter in the OpenAlex database.
- The publication has appeared in a source that is classified as journal, book, book series, or conference proceedings in the OpenAlex database.
- The publication has at least one reference and at least one author with an affiliation in the OpenAlex database.
- The publication has not been retracted.
Publications that do not meet the above criteria are not considered.
Universities do not need to apply to be included in the Leiden Ranking Open Edition. The universities included in the Leiden Ranking Open Edition are selected by CWTS according to the procedure described above. Universities do not need to provide any input themselves.
Data quality
For the assignment of publications to universities, the Leiden Ranking Open Edition relies on OpenAlex data. OpenAlex links affiliation strings to ROR identifiers. The linking of affiliation strings to ROR identifiers is a difficult task and not free of errors. The assignment of publications to universities has not been verified and approved by the universities themselves.
Two types of errors are possible in assigning publications to universities. On the one hand, there may be false positives, which are publications that have been assigned to a university while in fact they do not belong to the university. On the other hand, there may be false negatives, which are publications that have not been assigned to a university while in fact they do belong to the university. Both types of errors occur, but in general there are substantially more false negatives than false positives. One reason for this is that affiliation data is missing for a small share of the publications in OpenAlex. This blog post presents a comparison between the approaches for linking publications to universities used in the Leiden Ranking Open Edition and in the traditional Leiden Ranking.
