The CWTS Leiden Ranking Open Edition 2023 includes 1411 universities worldwide. These are the same universities that are also included in the traditional Leiden Ranking 2023. As discussed below, a sophisticated methodology is employed to assign publications to universities.

Research Organization Registry (ROR) and affiliated organizations

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 organizations. 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 organizations.

For the CWTS Leiden Ranking Open Edition we use the relationships between universities and their affiliated organizations 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 organizations:

  1. Component
  2. Joint research facility or organization
  3. 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.

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. It is also important to emphasize that in general 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.