Further - Excerpt from paper by Ladwig JP, Sommese AJ. "Using cited half-life to adjust download statistics " College & Research Libraries 66(6): 527-542, November 2005.

[Eugene Garfield]

Ladwig JP, Sommese AJ. "Using cited half-life to adjust download
statistics " College & Research Libraries 66(6): 527-542, November 2005.


J. Parker Ladwig : ladwig.1 at nd.edu
A.J. Sommese     : sommese at nd.edu

EXCERPT FROM FINAL REMARKS :
....

No model is perfect, but the half-life model fits the citation data
surprisingly well. The goal was to improve the method used to evaluate the
raw download figures. Indeed, the proposed model will still undercount the
same areas that the raw download figures undercount, but the undercounting
will be proportional across disciplines and less severe.

It is important when evaluating whether approximations are "acceptable" to
keep the goal in mind. The goal is a reduction in undercounting, and
undercounting is much more severe for long half-life journals with short
electronic runs available.

For example, look at a journal with a very long half-life, Mathematische
Annalen.  According to JCR, this respected mathematics journal has a haIf-
life of > 10 years with an electronic run of nine years. The ISI database
does not give the exact half-life; however, only 24.2 percent of citations
are from articles published between 1994 and 2003. Using Ieast-squares
minimization, the "ISI half-life" is 23.2 years. Math Ann. moves from a $46
CPU in table 4 to a $23 CPU in tabIe 5 and to a $17 CPU in table 6.

As time passes, the electronic runs of journals will increase, and there
will be sufficient year-to-year raw download figures to make reasonable
extrapolations. Thus, the model for adjusting downloads that this article
proposes will become less urgent.