Fields

The CWTS Leiden Ranking Open Edition 2023 provides statistics not only at the level of science as a whole but also at the level of the following five main fields of science:

• Biomedical and health sciences
• Life and earth sciences
• Mathematics and computer science
• Physical sciences and engineering
• Social sciences and humanities

As discussed below, these five main fields are defined based on large number of micro-level fields.

Algorithmically defined main fields

Each publication of a university belongs to one, or sometimes to more than one, of the above main fields. If a publication belongs to more than one main field, the publication is assigned fractionally to each of the main fields. For instance, a publication belonging to two main fields is assigned to each of the two fields with a weight of 1 / 2 = 0.5.

Publications are assigned to the five main fields using an algorithmic approach. Traditionally, fields of science are defined by sets of related journals. This approach is problematic especially in the case of multidisciplinary journals such as Nature, PLOS ONE, PNAS, and Science, which do not belong to one specific scientific field. The five main fields listed above are defined at the level of individual publications rather than at the journal level. In this way, publications in multidisciplinary journals can be properly assigned to a field.

Publications are assigned to main fields in the following three steps:

1. We start with 4521 micro-level fields of science. These fields are constructed algorithmically. Using a computer algorithm, each publication in OpenAlex is assigned to one of the 4521 fields. This is done based on a large-scale analysis of hundreds of millions of citation relations between publications.
2. We then determine for each of the 4521 micro-level fields the overlap with each of the 284 level 1 concepts defined in OpenAlex.
3. Each level 1 concept in OpenAlex has a link to one of the five main fields. Based on the link between level 1 concepts and main fields, we assign each of the 4521 micro-level fields to one or more of the five main fields. A micro-level field is assigned to a main field if at least 20% of the publications in the micro-level field cluster belong to subject categories linked to the main field.

After the above steps have been taken, each publication in OpenAlex has an assignment to a micro-level field, and each micro-level field in turn has an assignment to at least one main field. Combining these results, we obtain for each publication an assignment to one or more main fields.

Overview of micro-level fields

An interactive visualization of the micro-level fields, created using the VOSviewer software, is provided below. The visualization can be interpreted as follows:

• Each circle represents a micro-level field. Each field is labeled using a numerical identifier (integer value between 1 and 4521).
• The size of a circle indicates the number of publications in a micro-level field. The larger the circle, the larger the number of publications.
• The distance between two circles approximately indicates the relatedness of two micro-level fields, where the relatedness is determined by citation relations between the fields. In general, the smaller the distance between two circles, the stronger the micro-level fields are related to each other.
• The color of a circle indicates the main field to which a micro-level field belongs. Some micro-level fields belong to multiple main fields. In that case, the color of a circle is determined by the main field to which a micro-level field is most strongly linked. The color coding is as follows: Biomedical and health sciences (green), Life and earth sciences (yellow), Mathematics and computer science (purple), Physical sciences and engineering (blue), and Social sciences and humanities (red).
• In the interactive visualization, more detailed information about a micro-level field can be obtained by moving the mouse over the corresponding circle. The VOSviewer software will then list the main fields to which a micro-level field belongs, the five journals with the largest number of publications in a micro-level field, and five characteristic terms extracted from the titles of the publications in a micro-level field.

It should be noted that the micro-level fields play an important role in the calculation of the field-normalized impact indicators in the Leiden Ranking.

More information

This blog post discusses the approach taken to construct the micro-level fields. For more information on the methodology for the algorithmic construction of the micro-level fields, we refer to a paper by Waltman and Van Eck (2012). The methodology makes use of the Leiden algorithm. This algorithm is documented in a paper by Traag et al. (2019).

• Waltman, L., & Van Eck, N.J. (2012). A new methodology for constructing a publication-level classification system of science. Journal of the American Society for Information Science and Technology, 63(12), 2378–2392. (paper, preprint)
• Traag, V.A., Waltman, L., & Van Eck, N.J. (2019). From Louvain to Leiden: Guaranteeing well-connected communities. Scientific Reports, 9, 5233. (paper, preprint)