The Thomson Reuters journal impact factor (JIF) varies between disciplines and research fields and therefore there may be even major differences among the journals awarded the same Level by the various Panels (e.g., the Chemistry Panel and the Materials Technology Panel). JIF is not necessarily comparable even within the fields represented by one single Panel, as it may depend on the size of the field, the reference practices in use and several other factors.
In fact, the Panels have been instructed to pay attention to the share of different research fields included in the higher levels. As a consequence, Level 1 journals evaluated by one single Panel may have a higher JIF than certain Level 2 journals. Certain Panels have given more weight to JIFs compared to other impact and quality indicators provided as a support to the Panel’s evaluation work (Source Normalized Impact per Paper (SNIP), SCImago Journal Rank (SJR), European Reference Index for the Humanities (ERIH) as well as the Norwegian and Danish classifications).
Citation indicators also vary according to article types. Review articles gain more citations on average, compared to original articles, and therefore review journals have relatively high impact factors. Panels have been asked to make sure that Levels 2 and 3 do not include disproportionate numbers of journals with review articles. This means that review journals with a higher JIF may have a lower rating than journals with a lower JIF publishing original research.