Table 2.

Assessment of the quality of the studies included in the meta-analysis (Newcastle–Ottawa Scale)

Domain	Item	Kreder and Armstrong [22]	Skaggs et al. [23]	Skaggs et al. [24]
Selection (maximum of 4 stars)	Representativeness of the exposed cohort	*	*	*
	Selection of the unexposed cohort	*	*	*
	Ascertainment of exposure	*	*	*
	Demonstration that outcome of interest was not present at start of study	*	*	*
Comparability (maximum of 2 stars)	Comparability of cohorts on the basis of the design or analysis	Nil	Nil	Nil
Outcome (maximum of 3 stars)	Assessment of outcome	*	*	*
	Was follow-up long enough for outcome to occur?	*	*	*
	Adequacy of follow-up of cohorts	*	*	*

Maximum number of stars is 9 for the three domains