Authors:Pedro Albarrán, Juan A. Crespo, Ignacio Ortuño, and Javier Ruiz-Castillo
This paper studies evidence from Thomson Scientific (TS) about the citation process of 3.7 million articles published in the period 1998–2002 in 219 Web of Science (WoS) categories, or sub-fields. Reference and citation distributions have very different characteristics across sub-fields. However, when analyzed with the Characteristic Scores and Scales (CSS) technique, which is replication and scale invariant, the shape of these distributions over three broad categories of articles appears strikingly similar. Reference distributions are mildly skewed, but citation distributions with a 5-year citation window are highly skewed: the mean is 20 points above the median, while 9–10% of all articles in the upper tail account for about 44% of all citations. The aggregation of sub-fields into disciplines and fields according to several aggregation schemes preserve this feature of citation distributions. It should be noted that when we look into subsets of articles within the lower and upper tails of citation distributions the universality partially breaks down. On the other hand, for 140 of the 219 sub-fields the existence of a power law cannot be rejected. However, contrary to what is generally believed, at the sub-field level the scaling parameter is above 3.5 most of the time, and power laws are relatively small: on average, they represent 2% of all articles and account for 13.5% of all citations. The results of the aggregation into disciplines and fields reveal that power law algebra is a subtle phenomenon.
Authors:Pedro Albarrán, Juan Crespo, Ignacio Ortuño, and Javier Ruiz-Castillo
In this paper, scientific performance is identified with the impact that journal articles have through the citations they
receive. In 15 disciplines, as well as in all sciences as a whole, the EU share of total publications is greater than that
of the U.S. However, as soon as the citations received by these publications are taken into account the picture is completely
reversed. Firstly, the EU share of total citations is still greater than the U.S. in only seven fields. Secondly, the mean
citation rate in the U.S. is greater than in the EU in every one of the 22 fields studied. Thirdly, since standard indicators—such
as normalized mean citation ratios—are silent about what takes place in different parts of the citation distribution, this
paper compares the publication shares of the U.S. and the EU at every percentile of the world citation distribution in each
field. It is found that in seven fields the initial gap between the U.S. and the EU widens as we advance towards the more
cited articles, while in the remaining 15 fields—except for Agricultural Sciences—the U.S. always surpasses the EU when it
counts, namely, at the upper tail of citation distributions. Finally, for all sciences as a whole the U.S. publication share
becomes greater than that of the EU for the top 50% of the most highly cited articles. The data used refers to 3.6 million
articles published in 1998–2002, and the more than 47 million citations they received in 1998–2007.