Table 2. Assessment of the quality of the eleven studies.

	Aichinger 2012(9)	Blaivas 2001(10)	Breitkreutz 2010(11)	Flato 2015(13)	Chardoli 2012(12)	Salen 2001(16)	Salen 2005(4)	Tayal 2003(17)	Gaspari 2016(14)	Kim 2016(15)	Tomruk 2012(18)
Representativeness of the exposed cohort	*	*	*	*	*	*	*	*	*	*	*
Selection of the nonexposed cohort	*	*	*	*	*	*	*	*	*	*	*
Ascertainment of exposure	*	*	*	*	*	*	*	*	*	*	*
Demonstration that outcome of interest was not present at start of study	*	*	*	*	*	*	*	*	*	*	*
Comparability of cohorts on the basis of the design or analysis	**	-	-	**	-	-	-	-	**	**	**
Assessement of outcome	*	*	*	*	*	*	*	*	*	*	*
Was follow-up long enough for outcomes to occur	*	*	*	*	*	*	*	*	*	*	*
Adequacy of follow up of cohort	*	*	*	*	*	*	*	*	*	*	*

*: One star

**: Two stars.