Fields

The CWTS Leiden Ranking 2024 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 4234 micro-level fields of science. These fields are constructed algorithmically. Using a computer algorithm, each publication in Web of Science is assigned to one of the 4234 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 4234 micro-level fields the overlap with each of the 254 journal subject categories defined in Web of Science (excluding the Multidisciplinary Sciences subject category).
3. Each subject category in Web of Science has been linked to one of the five main fields. Based on the link between subject categories and main fields, we assign each of the 4234 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 25% of the publications in the micro-level field belong to subject categories linked to the main field.

After the above steps have been taken, each publication in Web of Science 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.

The link between subject categories and main fields can be found in this Excel file.

Overview of micro-level fields

Information on the 4234 micro-level fields is available in this Excel file. For each micro-level field, the file provides the following information:

• Numerical identifier (integer value between 1 and 4234).
• Number of publications in a micro-level field (2000–2023).
• List of the main fields to which a micro-level field belongs.
• List of the five journals with the largest number of publications in a micro-level field.
• Labeling obtained using a large language model that describes the topic of the a micro-level field (short label, long label, list of keywords, summary, and Wikipedia page).
• List of five characteristic terms extracted from the titles of the publications in a micro-level field.

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 4234).
• 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.

Assignment of publications to micro-level fields

CWTS and Clarivate have jointly decided to make the definitions of the micro-level fields publicly available. For each publication in Web of Science in the period 2006–2022 (article and review document types only), the Web of Science accession number (also known as the UT code) is made available along with a link to the micro-level field to which the publication has been assigned. Also, for each publication it is indicated whether the publication has been classified as a core or a non-core publication. If you want to get access to the definitions of the micro-level fields, please briefly tell us about the purpose for which you want to use the field definitions. To do so, please fill out the form below. CWTS and Clarivate will not share your data with other parties. The definitions of the micro-level fields are made available for non-commercial use. Redistribution and commercial use are not allowed.

More information

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)