Created by Professor Henk F. Moed at Centre for Science and Technology Studies (CTWS), 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 was developed using Elsevier (Scopus) citation data.
An example of CWTS Journal Indicators in the Education field from 1999 sorted by SNIP below:
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.