Citations occur in a given context, that is, in a given source, discipline, database, and year. All these variables are taken into account when calculating citations for a journal. This is key to understanding why journal rankings differ many times. This is because the methodologies for the calculation are different, as the citations are those represented in a given source (Web of Science, Scopus, etc.). This, together with using different bibliometric indicators, makes the rankings of journals vary. See below for a list of the most prominent journal rankings.