Source normalized impact per paper (SNIP) corrects for differences in publication rated among disciplines. It weighs citations to a journal based on the number of citations in that field. So disciplines with smaller publication rates can be compared to ones with higher rates. It's defined as the ratio of a journals citation count per paper and the citation potential (average length of lists of reference lists in a field) for the journals subject field.
Citation potential is shown to vary not only between journal subject categories – groupings of journals sharing a research field – or disciplines (e.g., journals in mathematics, engineering and social sciences tend to have lower values than titles in life sciences), but also between journals within the same subject category. For instance, basic journals tend to show higher citation potentials than applied or clinical journals, and journals covering emerging topics higher than periodicals in classical subjects or more general journals. SNIP corrects for such differences.
CWTS Journal Indicators provides free access to bibliometric indicators on scientific journals. Indicators are available for over 20,000 journals indexed in the Scopus database and include the number of publications per year, average number of citations per publication, the source normalized impact per publication, and percentage of self citations.