  Bibliometric methods are used in multiple fields for a variety of purposes,
namely for research evaluation. Most bibliometric analyses have in common their
data sources: Thomson Reuters' Web of Science (WoS) and Elsevier's Scopus. This
research compares the journal coverage of both databases in terms of fields,
countries and languages, using Ulrich's extensive periodical directory as a
base for comparison. Results indicate that the use of either WoS or Scopus for
research evaluation may introduces biases that favor Natural Sciences and
Engineering as well as Biomedical Research to the detriment of Social Sciences
and Arts and Humanities. Similarly, English-language journals are
overrepresented to the detriment of other languages. While both databases share
these biases, their coverage differs substantially. As a consequence, the
results of bibliometric analyses may vary depending on the database used.
