  This paper explores a new indicator of journal citation impact, denoted as
source normalized impact per paper (SNIP). It measures a journal's contextual
citation impact, taking into account characteristics of its properly defined
subject field, especially the frequency at which authors cite other papers in
their reference lists, the rapidity of maturing of citation impact, and the
extent to which a database used for the assessment covers the field's
literature. It further develops Eugene Garfield's notions of a field's
'citation potential' defined as the average length of references lists in a
field and determining the probability of being cited, and the need in fair
performance assessments to correct for differences between subject fields. A
journal's subject field is defined as the set of papers citing that journal.
SNIP is defined as the ratio of the journal's citation count per paper and the
citation potential in its subject field. It aims to allow direct comparison of
sources in different subject fields. 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. Its strengths and limitations are critically discussed, and
suggestions are made for further research. All empirical results are derived
from Elsevier's Scopus.
