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Journals maps and local impact factors

    Comment on:
    Richard Monastersky, The Number That's Devouring Science,
    Chronicle of Higher Education, October 14, 2005

Monasterky's article lists a number of problems with the ISI-impact factor.
However, he fails to mention that the average impact factors vary among
fields of science. For example, impact factors in toxicology are
considerably lower than in immunology. This may be contributing to the
concern over the use of these types of measures.  Furthermore, Bensman
recently showed that more than with the impact factor faculty usage and
appreciation of journals correlates with the total citations given to a
journal. Citations can be considered a measure of a journal's prestige,
while the impact factors follow the development of the fields at the
frontiers of research.

A fix to these problems might be a discipline-specific impact factor.
However, unambiguous clustering of the aggregated journal-journal citation
matrix into disciplines and specialties is impossible because the various
subsets overlap for very different reasons such as communalities in the
subject matter, methods, nationality, language, type of publisher or
purpose. Each journal has its own unique environment created in the acts of
citing and being-cited. Journals also differ in terms of their
within-journal ("self-citation") rates.

These challenges recently led me to take a different tact.  Using ISI's
Journal Citation Reports, I created the raw materials to make maps of the
citation neighborhoods of all the journals. The freeware program Pajek can
be used for the visualization. Clustering algorithms are available within
Pajek for differently colouring the visualizations; the input files are
available at . The contributions
to the total number of citations in this local environment can be considered
as a local impact factor. This local impact can additionally be corrected
for within-journal citations. I used the horizontal axis of the node for
this corrected local impact, while the vertical axis is used for the local
impact including self-citations. All values are expressed as percentages in
order to control for differences in citation behaviour among fields.

The advantages of this local impact factor are that (1) the normalization on
the total citations in the relevant citation environment is more indicative
of the intellectual status of a journal than an average normalized over the
number of publications like the impact factor of ISI; (2) the evaluation can
be made for each journal in the ISI-set and related to the journal's
specific citation environment; (3) the correction for within-journal
citations is available both numerically and from the visualizations.
Furthermore, this information is freely available on the internet.


Loet Leydesdorff
Amsterdam School of Communications Research (ASCoR)
Kloveniersburgwal 48, 1012 CX Amsterdam
Tel.: +31-20- 525 6598; fax: +31-20- 525 3681
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The Knowledge-Based Economy: Modeled, Measured, and Simulated

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