This paper studies the role of extremely highly cited articles in two instances: the measurement of
citation inequality, and mean citation rates. Using a dataset, acquired from Thomson Scientific, consisting of
4.4 million articles published in 1998-2003 in 22 broad fields with a five-year citation window, the main results
are the following. Firstly, both within each of 22 broad fields and in the all-sciences case, citation inequality is
strongly affected by the presence of a handful of extreme observations, particularly when it is measured by
citation inequality indices that are very sensitive to citation differences in the upper tail of citation distributions.
Secondly, the impact of extreme observations on citation averages is generally much smaller. The concluding
Section includes some practical lessons for students of citation inequality and/or users of high-impact
indicatorsThe authors acknowledge financial support by Santander Universities Global Division of Banco Santander. Ruiz-
Castillo also acknowledges financial help from the Spanish MEC through grant ECO2011-2976
