Table 2.

Number of primary studies for each literature repository and search string.

Literature Repositories	Search Strings (SS)
	SS1	SS2	SS3	SS4	SS5	SS6	SS7	SS8	SS9	SS10	SS11	SS12	SS13	SS14	SS15	SS16
IEEE Xplore	66	66	7	13	3	50	50	3	6	1	4	5	46	53	12	12
ACM Digital Library	34	34	5	13	13	33	33	1	13	13	25	30	68	81	2	2
Science Direct	34	34	25	25	25	41	41	30	30	30	1	1	3	3	3	3
Springer	52	52	29	7	7	75	75	29	9	9	3	3	2	2	2	2