  Journal metrics are employed for the assessment of scientific scholar
journals from a general bibliometric perspective. In this context, the Thomson
Reuters journal impact factors (JIF) are the citation-based indicators most
used. The 2-year journal impact factor (2-JIF) counts citations to one and two
year old articles, while the 5-year journal impact factor (5-JIF) counts
citations from one to five year old articles. Nevertheless, these indicators
are not comparable among fields of science for two reasons: (i) each field has
a different impact maturity time, and (ii) because of systematic differences in
publication and citation behaviour across disciplines. In fact, the 5-JIF
firstly appeared in the Journal Citation Reports (JCR) in 2007 with the purpose
of making more comparable impacts in fields in which impact matures slowly.
However, there is not an optimal fixed impact maturity time valid for all the
fields. In some of them two years provides a good performance whereas in others
three or more years are necessary. Therefore, there is a problem when comparing
a journal from a field in which impact matures slowly with a journal from a
field in which impact matures rapidly. In this work, we propose the 2-year
maximum journal impact factor (2M-JIF), a new impact indicator that considers
the 2-year rolling citation time window of maximum impact instead of the
previous 2-year time window. Finally, an empirical application comparing 2-JIF,
5-JIF, and 2M-JIF shows that the maximum rolling target window reduces the
between-group variance with respect to the within-group variance in a random
sample of about six hundred journals from eight different fields.
