Changes to SNIP & SJR metrics
Explanations of two citation indicators used to measure a journal's impact
By Sarah Huggett | Publishing Information Manager, Elsevier Posted on 27 February 2013
In recent years, computational advances have contributed to acceleration in the field of bibliometrics. While for a long time the journal evaluation landscape was somewhat dominated by a scarcity of measures, there are now many journal metrics available, providing a varied and more integral picture of journal impact . Editors may find these useful to compare their journal to competitors in various systematic ways.
Scopus features two such citation indicators to measure a journal's impact; SNIP (Source Normalised Impact per Paper) and SJR (SCImago Journal Rank). These indicators use the citation data captured in the Scopus database to reveal two different aspects of a journal's impact:
- SNIP takes into account the field in which a journal operates, smoothing differences between field-specific properties such as the number of citations per paper, the amount of indexed literature, and the speed of the publication process.
- SJR takes into account the prestige of the citing journal; citations are weighted to reflect whether they come from a journal with a high or low SJR.
These two indicators use a three-year window, are freely available on the web  and are calculated for all journals indexed in the Scopus database. The metrics have article-type consistency, i.e. only citations to and from scholarly papers are considered.
Some editors may have noticed changes to both SNIP and SJR values for their and other journals around mid-October 2012. These changes were introduced to make the metrics more intuitive and easy to understand. Following these improvements, the values are now computed and released once a year in the summer.
Further information on both metrics is available on the Journal Metrics website.
SNIP: How does it work?
SNIP was developed by Henk Moed, Senior Scientific Advisor at Elsevier, who was then part of the CWTS bibliometrics group at the University of Leiden, The Netherlands. It is a ratio, with a numerator and a denominator. SNIP's numerator gives a journal's raw impact per paper (RIP). This is simply the average number of citations received in a particular year (e.g. 2013) by papers published in the journal during the three preceding years (e.g. 2010, 2011 and 2012).
SNIP's denominator, the Database Citation Potential (DCP) is calculated as follows. We know that there are large differences in the frequency at which authors cite papers between various scientific subfields. In view of this, for each journal an indicator is calculated of the citation potential in the subject field it covers. This citation potential is included in SNIP's denominator.
SNIP is RIP divided by DCP.
In October 2012, the following changes were applied:
- A different averaging procedure is now used in the calculation of the denominator to reduce the impact of outliers.
- A correction factor has been introduced to the weighting of citations from journals with low numbers of references.
- The new calculation results in a SNIP average score for all journals in Scopus to approximately equal one.
Further details are available on ScienceDirect .
Dr Ludo Waltman, Researcher at the Centre for Science and Technology Studies (CWTS) of Leiden University, commented: “SNIP allows the impact of journals to be compared across fields in a fair way, and has been updated following the most recent insights in the fields of bibliometrics and scientometrics. The recent changes ensure the most balanced treatment of journals from different fields, with minimal implications for users.”
SJR: How does it work?
SJR was developed by the SCImago research group from the University of Granada, dedicated to information analysis, representation and retrieval by means of visualization techniques.
SJR looks at the prestige of a journal, as indicated by considering the sources of citations to it, rather than its popularity as measured simply by counting all citations equally. Each citation received by a journal is assigned a weight based on the SJR of the citing journal. A citation from a journal with a high SJR value is worth more than a citation from a journal with a low SJR value.
In October 2012, the following changes were applied:
- A heavier weighting of the more prestigious citations that come from within the same field, or those closely related.
- A compensating factor to overcome the decrease of prestige scores over time as the number of journals increases.
- A more readily understandable scoring scale with an average of one.
Further details are available on ScienceDirect.
 Bollen J, Van de Sompel H, Hagberg A, Chute R (2009) A Principal Component Analysis of 39 Scientific Impact Measures. PLoS ONE 4(6): e6022. doi:10.1371/journal.pone.0006022
 Waltman L, van Eck N J, van Leeuwen T N, Visser M S; Some modifications to the SNIP journal impact indicator, Journal of Informetrics, Volume 7, Issue 2, April 2013, http://dx.doi.org/10.1016/j.joi.2012.11.011
 Guerrero-Bote V P, Moya-Anegón F; A further step forward in measuring journals journals' scientific prestige: The SJR2 indicator, Journal of Informetrics, Volume 6, Issue 4, October 2012, http://dx.doi.org/10.1016/j.joi.2012.07.001