Table 2.

Characteristics of studies included in published meta-analyses that were not retrieved by citation-based literature search at each selection threshold

	All co-citations	All co-cited >1	Frequently co-cited
Retrieved	106	92	80
Missed	9 (5)	14 (6)	12 (7)
Abstract	2 (0)	0 (0)	1 (0)
Non-English language	1 (0)	6 (1)	0 (0)
Old publication (<1975)	2 (2)	2 (0)	1 (0)
Recent publication (2014)	2 (2)	1 (1)	0 (0)
Other	2 (1)	5 (5)	10 (7)
Total	115	106	92

Legend: The ten meta-analyses included 115 studies, of which 106 were retrieved by our search. Of those, 92 were co-cited more than once and 80 appeared in the list of frequently co-cited articles. The headings of the table refer to the thresholds presented in Table 1. The numbers in parentheses indicate how many articles had direct connections with other articles in the meta-analysis, because they were either citing or cited by those articles. These numbers indicate whether the articles could have been found by adding a search for direct citations, as was done in Study 2. For example, five of the nine studies that were missed in the first selection were citing or cited by other articles included in the meta-analysis