Fig. 1

Overview of the search method. Circles represent articles and lines are the citations between them. Arrows indicate the direction of the citation. Bold circles represent the query articles that are used to begin a search. Numbers in the circles indicate the co-citation or citation counts. Dashed circles represent articles that will not be screened for eligibility if the screening threshold is higher than one. Figure is adapted from [7] (distributed under the Creative Commons Attribution 4.0 International License)