Fig. 1

Overview of the search method. a Indirect citations (co-citations). Bold circles represent articles known at the beginning of the search. Squares represent citing articles; the articles on their reference lists (co-citing articles) are represented by circles. Numbers within circles indicate the number of times an article is co-cited (dashed circles represent articles co-cited only once). b. Direct citations. Bold circles represent articles known at the beginning of the search. Dashed squares represent citing articles; dashed circles represent articles on the known articles’ reference lists. Numbers within dashed squares and circles indicate the number of times an article cites or is cited by a known article