Table 2. Assessment of quality of studies by Newcastle-Ottawa Scale.

Studies	Selection			Comparability			Outcome			Total
	1	2	3	4	5A	5B	6	7	8
Pastore 2015[21]	*	*		*	*	*	*	*	*	8
Grotenhuis 2015[10]	*	*		*	*	*	*	*	*	8
Wyszynski 2014[24]	*	*		*	*	*	*	*	*	8
Kim 2014[17]	*	*		*	*	*	*	*	*	8
Serretta 2013[11]	*	*	*	*	*	*	*	*	*	9
Rink 2013[7]	*	*		*	*	*	*	*	*	8
Rink 2013[8]	*	*		*	*	*	*	*	*	8
Da Silva 2013[16]	*	*		*	*	*	*	*	*	8
Lee 2012[12]	*	*		*	*	*	*	*	*	8
Sfakianos 2011[18]	*	*	*	*			*	*	*	7
Chade 2010[19]	*	*			*	*	*	*	*	7
Ahirwar 2008[23]	*	*	*	*	*		*	*	*	8
Chen 2007[9]	*	*	*	*	*	*	*	*	*	9
Leibovici 2005[20]	*	*	*	*			*	*	*	7
Allard 1995[22]	*	*	*		*		*	*	*	7

1 = Representativeness of the exposed cohort;

2 = Selection of the non-exposed cohort;

3 = Ascertainment of exposure;

4 = Demonstration that outcome of interest was not present at start of study;

5 = Comparability of cohorts on the basis of the design or analysis (a. study controls for age; b. study controls for any additional factor);

6 = Assessment of outcome;

7 = Follow-up long enough for outcomes to occur;

8 = Adequacy of follow up of cohorts.