Pernbeger's (2004) finding that download counts (what we call "usage
impact") of British Medical Journal articles predict citation
counts ("citation impact") for those articles in subsequent
years confirms what Tim Brody's online usage/citation correlator
http://citebase.eprints.org/analysis/correlation.php has been
demonstrating for several years now across a number of areas in physics
and mathematics (Brody & Harnad 2004, in prep.): There is a significant
correlation between downloads today and citations two years later.

This correlation has two immediate implications:

(1) Download counts can be used as early performance indicators for
papers and authors, even before their impact is reflected in citation
counts: http://citebase.eprints.org/ ( Hitchcock et al. 2003).

(2) Enhancing usage impact is yet another reason for authors to
provide open access to their articles by self-archiving them.

References

Brody, T. & Harnad, S. (2004, in prep.) Using Web Statistics as a
predictor of Citation Impact.
http://www.ecs.soton.ac.uk/~harnad/Temp/timcorr.doc

Harnad, S. & Brody, T. (2004) Comparing the Impact of Open Access (OA)
vs. Non-OA Articles in the Same Journals, D-Lib Magazine 10 (6) June
http://www.dlib.org/dlib/june04/harnad/06harnad.html

Hitchcock, Steve; Woukeu, Arouna; Brody, Tim; Carr, Les; Hall, Wendy and
Harnad, Stevan. (2003) Evaluating Citebase, an open access Web-based