Impact per publication measures the ratio of citations in a year to scholarly papers published in the three previous years divided by the number of scholarly papers published in those same years. The IPP 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 chance of manipulation.
The IPP is not normalized for the subject field and therefore gives a raw indication of the average number of citation a publication published in the journal will likely receive. When normalized for the citations in the subject field, the raw impact per publication becomes the source normalized impact per paper (SNIP). Note that in the context of the calculation of SNIP, the raw impact per publication is usually referred to as RIP.
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.