Publication: Differences in citation impact across scientific fields
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2012-06
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Abstract
This paper has two aims: (i) to introduce a novel method for measuring which part of overall
citation inequality can be attributed to differences in citation practices across scientific fields, and (ii) to
implement an empirical strategy for making meaningful comparisons between the number of citations
received by articles in the 22 broad fields distinguished by Thomson Scientific. The paper is based on a model
in which the number of citations received by any article is a function of the article’s scientific influence, and
the field to which it belongs. The model includes a key assumption according to which articles in the same
quantile of any field citation distribution have the same degree of citation impact in their respective field. Using
a dataset of 4.4 million articles published in 1998-2003 with a five-year citation window, we find that
differences in citation practices between the 22 fields account for about 14% of overall citation inequality.
Our empirical strategy for making comparisons of citation counts across fields is based on the strong
similarities found in the behavior of citation distributions over a large quantile interval. We obtain three main
results. Firstly, we provide a set of exchange rates to express citations in any field into citations in the all-fields
case. (This can be done for articles in the interval between, approximately, the 71st and the 99th percentiles of
their citation distributions). The answer is very satisfactory for 20 out of 22 fields. Secondly, when the raw
citation data is normalized with our exchange rates, the effect of differences in citation practices is reduced to,
approximately, 2% of overall citation inequality in the normalized citation distributions. Thirdly, we provide
an empirical explanation of why the usual normalization procedure based on the fields’ mean citation rates is
found to be equally successful.