About Source Normalized Impact per Paper (SNIP)
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 is defined as the ratio of a journal's raw impact per publication and the citation potential in its subject field. This allows for evaluation of a journal compared to its competition and provides more contextual information, giving a better picture of the impact depending on the citation behavior in the field.
The Impact per Publication measures the ratio of citations in a year (Y) to scholarly papers published in the three previous years (Y-1, Y-2, Y-3) divided by the number of scholarly papers published in those same years (Y-1, Y-2, Y-3). The Impact per Publication metric is using a citation window of three years which is considered to be the optimal time period to accurately measure citations in most subject fields. Taking into account the same peer-reviewed scholarly papers only in both the numerator and denominator of the equation provides a fair impact measurement of the journal and diminishes the change of manipulation.
SJR is a prestige metric based on the idea that not all citations are the same. With SJR, the subject field, quality and reputation of the journal have a direct effect on the value of a citation and the impact that journal makes. In addition the prestige of a citation is weighted over all citations handed out to that journal. SJR is a size-independent indicator and it ranks journals by their 'average prestige per article' and can be used for journal comparisons in science evaluation processes.
From the About Journal Metrics Page, retrieved on August 10, 2015
"Our Journal Metrics apply to over 20,000 journals, proceedings and book series and:
Get Started with Journal Metrics by searching via Journal Title Keyword. Results can be sorted by Year, SNIP, IPP, and SJR Ranking, in descending or ascending order.
Click, Search and you will see the results which list the SNIP, IPP and SJR for the last three years of publication.