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SNIP — a unique method for journal evaluation

2025-01-13 16:29
The influence of journals can be rated using special metrics. For example, the Impact factor (JIF) is calculated based on Web of Science data, or SCIMago Journal Rank (SJR) and CiteScore based on Scopus data, which we discussed earlier. Today we will consider one more tool for assessing journals - the Source Normalized Impact per Paper (SNIP) indicator.
The SNIP indicator uses the method of normalizing citations by subject depending on the source of references to eliminate the differences in citation practice among scientific fields. The advantage of this approach is that it does not require a subject classification system with some clearly defined boundaries, as does the quartile approach. Thus, this metric provides a comparison of citation rates between journals from different subject areas. The indicator was proposed by the Dutch scientometrician Henk Moed in 2010, and an updated version of the metric was published in 2012.
The SNIP value of a journal is the result of division, where the numerator is the average citation rate of an article in the assessed journal, and the denominator is the average length of the reference lists of the papers, which cite this journal. Thus, the longer the reference list in the citing article, the lower the value of the link obtained from this article. For example, in Humanities, reference lists are shorter, so the weight of citations from such works is higher than in Physics, where reference lists are longer. The calculation of the updated metric is shown in the picture below. A more detailed explanation of the calculation can be found in the original article.
The indicator is calculated based on the data from Scopus by Elsevier. For new journals indexed in the database, the SNIP metric is normally assigned in June within the following annual calculation period, starting from the year they were added and for all the previous years. The calculated SNIP indicators for journals have been available since 1999. You can view the SNIP metric values using the open functions of the Scopus Preview, along with SJR and CiteScore.
The detailed information about the indicator methodology, the SNIP values ​​of journals, as well as the number of papers, percentage of self-citations and other indicators are presented directly on the website of the Center for Science and Technology Studies at Leiden University (CWTS).

Some details about the SNIP calculation

  • The time span, or "publication window" is a three-year period preceding the reporting year. The papers of the reporting year itself are not taken into account for the calculation;
  • Citations are only considered for the reporting year;
  • The calculation is possible only for articles, conference papers and reviews. Citations also are taken only from the documents of one of the three specific types mentioned.
The authors of the article claim that SNIP should be used with caution in the following cases:
  • SNIP does not see the difference between "Article" and "Review". Reviews are generally cited much more often than original research papers. Therefore, journals that publish many review articles usually have a higher SNIP value.
  • Journal self-citations. Some journals may attempt to unfairly increase their influence. SNIP does not adjust for this factor. However, the percentage of a journal self-citations is reported as a separate indicator in CWTS Journal Indicators. Please note that more voluminous journals can be expected to have a higher percentage of self-citations than journals with fewer publications.
  • The SNIP calculation is more reliable for more voluminous journals than for smaller ones with a limited number of publications. For this reason, CWTS Journal Indicators display by default the statistics only for the journals with at least 50 publications.
  • The distribution of citations across papers in a journal is typically uneven, with a large number of uncited and low-cited publications and only a small number of highly cited ones. Because of this asymmetry, the average citation rate of a journal is not very representative.
  • SNIP values ​​can sometimes be heavily influenced by one or more very highly cited publications. This will certainly result in wide stability intervals, so this parameter should also be taken into account when interpreting SNIP. The definition of the stability interval and the calculated values ​​are available from the links to the website.
  • Some journals (mostly from Arts and Humanities) are identified by the compilers as non-citing sources. That means that citations from these journals are ignored when calculating SNIP, however, the journals themselves may have a SNIP value.
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