Journals maps and local impact factors
Chronicle of Higher Education, November 18, 2005
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 led me recently 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 http://users.fmg.uva.nl/lleydesdorff/jcr04 . 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),
University of Amsterdam, Kloveniersburgwal 48,
1012 CX Amsterdam, The Netherlands