As journal metrics become more and more important for scientists and their institutions, Copernicus Publications decided to present the four most important ones. A short explanation is listed below. For further information please visit the individual websites of Thomson Reuters and SCOPUS.
It is important NOT to compare the different journal metrics, as the results stem from different calculations. As a comparison, the 2009 journal metrics for "Nature Geoscience" are listed:
The annual Journal Citation Report Impact Factor is a ratio between citations and recent citable items published. Thus, the impact factor of a journal is calculated by dividing the number of current year citations by the source items published in that journal during the previous two years.
The 5-year impact factor is calculated by applying the counted articles to the previous five years.
Source: Thomson Reuters
The 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.
The SCImago Journal Rank (SJR) is a prestige metric based on the idea that "all citations are not created equal". With SJR, the subject field, quality and reputation of the journal has a direct effect on the value of a citation.