VOSviewer software (version 1.6.16, released on November 25, 2020) was used for bibliometric analysis and to plot bibliometric networks . The terms were organized into clusters, with each cluster represented by a color. More important terms had larger circles than less important ones, and strongly related terms were positioned closer. Moreover, lines were drawn between items to indicate relations, with thicker lines indicating a stronger link between 2 items . Because VOSviewer software cannot identify singular and plural nouns, or different spellings of the same keyword, such keywords were excluded when plotting keyword co-occurrence graphs. Thus, for 2015, 99 keywords were analyzed, for , for 2020 -96 keywords.