Impact Factor, SJR, SNIP, and CiteScore

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The Journal Impact Factor (JIF) is defined as all citations to the journal in the current JCR year to items published in the previous two years, divided by the total number of scholarly items (these comprise articles, reviews, and proceedings papers) published in the journal in the previous two years.

The Journal Impact Factor takes into account the outbound cited references from any of the five journal and proceedings indexes in Web of Science (WoS):

  • Science Citation Index Expanded (SCIE)
  • Social Sciences Citation Index (SSCI)
  • Arts & Humanities Citation Index
  • Conference Proceedings Citation Index, Science edition
  • Conference Proceedings Citation Index, Social Science and Humanities edition
  • For each title in SCIE or SSCI (only these two indexes get JIFs), the citations it earns (among the outbound citations measured), are collected and summed.

This collection and summation takes into account the year of publication for (a) the outbound citation (=JCR year) and for (b) the item that has been cited. The Journal Impact Factor is restricted to a two-year window of interest for cited item publication year: one year prior to the JCR year (= year -1) and two years prior to the JCR year (= year -2).

For example, in the 2015 JCR, each Journal Impact Factor will measure the citations earned by a publication where the citing year is 2015, and the cited year is either 2013 or 2014.

The Journal Impact Factor has a simple formula:

Journal Impact Factor = (citations from JCR year to items in “year -2” + citations from JCR year to items in “year -1”)/ (citable items in “year -2” + citable items in “year -1”)

SCImago Journal Rank (SJR) measures weighted citations received by the serial. Citation weighting depends on subject field and prestige (SJR) of the citing serial.

SJR is weighted by the prestige of a journal. Subject field, quality, and reputation of the journal have a direct effect on the value of a citation.

SJR assigns relative scores to all of the sources in a citation network. Its methodology is inspired by the Google PageRank algorithm, in that not all citations are equal. A source transfers its own ‘prestige’, or status, to another source through the act of citing it. A citation from a source with a relatively high SJR is worth more than a citation from a source with a lower SJR.

Source Normalized Impact per Paper (SNIP) measures actual citations received relative to citations expected for the serial’s subject field.

It helps you make a direct comparison of sources in different subject fields.

SNIP takes into account characteristics of the source’s subject field, which is the set of documents citing that source. SNIP especially considers:

  • The frequency at which authors cite other papers in their reference lists
  • The speed at which citation impact matures
  • The extent to which the database used in the assessment covers the field’s literature

SNIP is the ratio of a source’s average citation count per paper and the citation potential of its subject field.

The citation potential of a source’s subject field is the average number of references per document citing that source. It represents the likelihood of being cited for documents in a particular field. A source in a field with a high citation potential tends to have a high impact per paper.

CiteScore measures average citations received per document published in the serial.  CiteScore is a simple way of measuring the citation impact of serial titles such as journals. Serial titles are defined as titles which publish on a regular basis (i.e. one or more volumes per year).

CiteScore calculates the average number of citations received in a calendar year by all items published in that journal in the preceding three years. The calendar year to which a serial title’s issues are assigned is determined by their cover dates, and not the dates that the serial issues were made available online. The method of calculation for CiteScore 2016 is illustrated below.

CiteScore 2017 counts the citations received in 2017 to documents published in 2014, 2015 or 2016, and divides this by the number of documents published in 2014, 2015 and 2016.

The Journal Impact Factor Quartile (Clarivate Analytics) is the quotient of a journal’s rank in category (X) and the total number of journals in the category (Y), so that (X / Y) = Percentile Rank Z.

Q1: 0.0 < Z ≤ 0.25

Q2: 0.25 < Z ≤ 0.5

Q3: 0.5 < Z ≤ 0.75

Q4: 0.75 < Z

How to compare journals among subject field?

You should not compare between subject fields using CiteScore. This metric is not field-normalized, and different publication and citation behavior of researchers in different fields affects the values, as well as differences in performance.

You can compare between subject fields in any of the following three ways:

  • Using the CiteScore percentile within a subject field. Journals that are in the top 10% of distinct fields have a similar citation impact on that field, even though this percentile is based on a CiteScore with a different absolute value
  • Use SNIP, which is a field-normalized metric
  • Use SJR, which is a field-normalized metric

How about self-citation handled?

The definition of self-citations that is relevant for CiteScore metrics is citations made by documents published in the serial title to other documents published in the same serial title. In almost all cases, these citations are a responsible and useful behavior to indicate related documents to readers; it is reasonable to expect related papers to be published in the same serial title. Consequently, journal self-citations are included in CiteScore metrics.

In Clarivate Analytics, Journal self-citation is a known aspect of referencing practice. Nearly every journal in the JCR-Science Edition in 2002 contains at least some reference to its own, previous literature. Examining the entire population of journals in the JCR-Science Edition, we can establish a criterion for an expected level of journal self-citation. Here we determine that a self-citation rate of 20% or less is characteristic of the majority of the high-quality science journals selected for coverage in Clarivate Analytics products.

We found that self-citation rate correlates only weakly with category size or number of assigned categories. Rather, self-citation is a characteristic of an individual journal’s interaction with the citing literature, and should only be considered at the level of the individual journal.

A relatively high self-citation rate can be due to several factors. It may arise from a journal’s having a novel or highly specific topic for which it provides a unique publication venue. A high self-citation rate may also result from the journal having few incoming citations from other sources. Journal self-citation might also be affected by sociological factors in the practice of citation. Researchers will cite journals of which they are most aware; this is roughly the same population of journals to which they will consider sending their own papers for review and publication. It is also possible that self-citation derives from an editorial practice of the journal, resulting in a distorted view of the journal’s participation in the literature. The consideration of self-citation can reveal journals with an excessive reliance on self-citation, unexplainable by any other characteristic of the journal.

For the majority of journals, low and moderate levels of self-citation are an expected part of their interaction with the literature. We studied the effect of self-citation on Impact Factor and ranking of journals in the Cell Biology category and found that there is little change to the relative rank of the top ten journals when self-citations are removed from consideration.

Citation represents a connection between two published articles. It is an article-level interaction. Ideally, authors will choose the most relevant works to cite, independently of the journal in which they were published. The JCR contains citations aggregated at the journal-level, and, while they do not show article-by-article practices, they can reveal cases where journal performance is distorted by a high rate of self-citation. Although it is not addressed in the current work, examination of a journal’s pattern of outgoing citation (derived from the Citing Journal data in the JCR) could also reveal a biased citation practice at the journal level. Cited Journal data are collected for each title from across the entire Clarivate Analytics Citation Database. A journal cannot directly affect the degree to which it is cited by other titles and so cannot affect its Cited Journal statistics. Citing Journal data, in contrast, are derived from material published in the title, and therefore can be revealing of a journal-level practice of self-citation.

The current study was based on the analysis of a single citing year of data. Because citation is a dynamic and on-going phenomenon, no one year of citation data is sufficient to define the self-citation practice of an individual journal. Several consecutive years of citation patterns are necessary to establish whether a journal is actively participating in the scientific communications in its field, or if it is relying primarily on self-citations for impact. Further, this study was limited to the JCR — Science Edition. Citation practices in social sciences differ from those in science, and were not included in this study.

The Cited Journal data in the JCR have always contained information on journal self-citation. In 2004, a new interface to the JCR on the Web® will present the cited and citing journal data graphically, and will specifically include display of journal self-citations and their contribution to the key citation metrics of Immediacy Index, Impact Factor and total citations. As journal self-citation data become accessible even to casual users of the JCR, it is important to understand these data in the context of citation practices throughout the population of journals in the Clarivate Analytics Citation Databases.

  1. Note: The practice of self-citation can be considered at many levels, including author self-citation, journal self-citation, and subject category self-citation. For the purposes of this study, “self-citation” will be used to refer only to journal self-citation as here defined.
  2. Journals that do not reference any of their own previous literature are defined as having a self-citation rate equal to zero. This can be either a practice of the journal itself, or a result of a title change. The previous title of the journal will appear in the JCR with no references processed in 2002, therefore no self-references in 2002. The new title referencing the previous title was not counted as self-citation for the purposes of this study. A title change can reflect a significant alteration to the content, scope, or editorial practices of a journal, and the relevance of new-title to previous title citation would need to be determined on a case-by-case basis.