  There exist ample demonstrations that indicators of scholarly impact
analogous to the citation-based ISI Impact Factor can be derived from usage
data. However, contrary to the ISI IF which is based on citation data generated
by the global community of scholarly authors, so far usage can only be
practically recorded at a local level leading to community-specific assessments
of scholarly impact that are difficult to generalize to the global scholarly
community. We define a journal Usage Impact Factor which mimics the definition
of the Thomson Scientific's ISI Impact Factor. Usage Impact Factor rankings are
calculated on the basis of a large-scale usage data set recorded for the
California State University system from 2003 to 2005. The resulting journal
rankings are then compared to Thomson Scientific's ISI Impact Factor which is
used as a baseline indicator of general impact. Our results indicate that
impact as derived from California State University usage reflects the
particular scientific and demographic characteristics of its communities.
