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

SNIP:

• 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 developed using Elsevier (Scopus) citation data.

The main differences between the indicators provided by CWTS Journal Indicators, in particular the IPP and SNIP indicators, and the Journal Impact Factor (JIF) are:

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

An example of CWTS Journal Indicators in Economics from 1999 sorted by SNIP below: