Table 1.

Levels of evidence by study

Level of evidence	Total number of studies	Included studies
1b	16	2, 3, 12, 13, 21, 25, 26, 27, 28, 30, 31, 36, 38, 39, 40, 41
2b	19	4, 5, 7a, 11, 15, 17, 18, 19, 22, 29, 32, 34, 35, 37, 42, 43, 44, 45, 47
4	3	7a, 9, 16

Level of study is determined by Oxford Centre for Evidence-based Medicine guidelines

^aStudy 7 is included twice because it involved 2 phases of different study design