For more than 40 years, the Institute for Scientific Information (ISI, now part of Thomson-Reuters), produced the only available database making possible citation analysis, the Web of Science (WoS). Now, another company, Reed-Elsevier, has created its own bibliographic database, Scopus, available since 2002. For those who perform bibliometric analysis and comparisons of countries or institutions, the existence of these two major databases raises the important question of the comparability and stability of rankings obtained from different data sources