  Bibliometric indicators can be determined by comparing specific citation
records with the percentiles of a reference set. However, there exists an
ambiguity in the computation of percentiles because usually a significant
number of papers with the same citation count are found at the border between
percentile rank classes. The present case study of the citations to the journal
Europhysics Letters (EPL) in comparison with all physics papers from the Web of
Science shows the deviations which occur due to the different ways of treating
the tied papers in the evaluation of the percentage of highly cited
publications. A strong bias can occur, if the papers tied at the threshold
number of citations are all considered as highly cited or all considered as not
highly cited.
