Table 5.

Level of evidence according to Oxford Centre for Evidence-Based Medicine

Study	Design	Level of evidence
Lubega et al. (2017) [37]	Prospective cohort study	4
Chang et al. (2011)	Prospective cohort study	4
Rojo et al. (2012) [35]	Retrospective case–control study	3b
Bashyal et al. (2009) [34]	Retrospective cohort study	4
Mc Cray et al. (1986) [33]	Retrospective case–control study	4
Bucher et al. (2011) [42]	Retrospective case–control study	3b
Meier et al. (2001) [38]	Randomized controled trial	2b
Berry et al. (1982) [31]	Randomized controled trial	2b