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Bibliometrics and Altmetrics: Measuring the Impact of Knowledge: SNIP (Source Normalized Impact per Paper)

DEFINITION: Source Normalized Impact per Paper (SNIP)

Source Normalized Impact per Paper (SNIP) measures contextual citation impact by weighting citations based on the total number of citations in a subject field. The impact of a single citation is given higher value in subject areas where citations are less likely, and vice versa.  Unlike the well-known journal impact factor, SNIP corrects for differences in citation practices between scientific fields, thereby allowing for more accurate between-field comparisons of citation impact. CWTS Journal Indicators also provides stability intervals that indicate the reliability of the SNIP value of a journal. 

Background

SNIP was created by Professor Henk F. Moed at Centre for Science and Technology Studies (CWTS), University of Leiden.

SNIP in 2009...

  • Measures contextual citation impact by ‘normalizing’ citation values.
  • Takes a research field’s citation frequency into account.
  • Considers immediacy - how quickly a paper is likely to have an impact in a given field.
  • Accounts for how well the field is covered by the underlying database.
  • Calculates without use of a journal’s subject classification to avoid delimitation.
  • Counters any potential for editorial manipulation.

SNIP was revised in 2012 to include these indicators:

  • P - the number of publications of a source in the past three years.
  • RIP - the raw impact per publication, calculated as the number of citations given in the present year to publications in the past three years divided by the total number of publications in the past three years. RIP is fairly similar to the well-known journal impact factor. Like the journal impact factor, RIP does not correct for differences in citation practices between scientific fields.
  • SNIP - the source normalized impact per publication, calculated as the number of citations given in the present year to publications in the past three years divided by the total number of publications in the past three years. The difference with RIP is that in the case of SNIP citations are normalized in order to correct for differences in citation practices between scientific fields. Essentially, the longer the reference list of a citing publication, the lower the value of a citation originating from that publication. A detailed explanation is offered in our scientific paper.
  • % self-citations - the percentage of self-citations of a source, calculated as the percentage of all citations given in the present year to publications in the past three years that originate from the source itself.

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Further Reading:

Differences with Journal Impact Factors

The main differences between the indicators provided by CWTS Journal Indicators, in particular the RIP and SNIP indicators, and the journal impact factor (JIF) can be summarized as follows:

  • Based on Scopus (RIP and SNIP) vs. based on Web of Science (JIF).
  • Correction for field differences (SNIP) vs. no correction for field differences (RIP and JIF).
  • Three years of cited publications (RIP and SNIP) vs. two years of cited publications (JIF).
  • Citations from selected sources and selected document types only (RIP and SNIP) vs. citations from all sources and document types (JIF).
  • Citations to selected document types only (RIP and SNIP) vs. citations to all document types (JIF).

More information

Technical Details of CWTS Journal Indicators

SNIP is just one type of measure created by Moed and his team.

Look in this PDF for the descriptions of the types of data represented in the Centre for Science and Technology Studies (CWTS) Journal Indicators at Leiden University.

Example of a Source Normalized Impact per Paper (SNIP) Search

To compare journals in a specific subject field, follow the steps:

          1. Go to CWTS Journal Indicators web site.

          2. Click on the Indicators tab.

          3. Make your selections from the drop down menus. In this search, the SNIP Indicators for journals in Psychology are shown below.

SNIP indicators for Psychology