TABLE 4.

Risk of bias assessments of Cohort studies based on New Castle ‐ Ottawa Quality Assessment Scale

Study	Representativeness of the exposed cohort	Selection of the non exposed cohort	Ascertainment of exposure	Outcome not present at the start of the study	Comparability of cases and controls	Assessment of outcome	Sufficient follow‐up time for outcomes to occur	Adequacy of follow‐up of cohorts	Total
Degidi et al. (2011), Oxby et al. (2015)	*	*	*	*	*	*	*	*	8
Oxby et al. (2015)	*	*	*	*	*	*	*	*	8
Siebers et al. (2010)	*	*	*	*	*	*	*		7
Simons et al. (2015)	*	*	*	*	*	*	*		7
Spies et al. (2015)	*	*	*	*	*	*	*	*	8

Thresholds for converting the Newcastle‐Ottawa scales to AHRQ standards (good, fair, and poor):

Good quality: 3 or 4 stars in selection domain AND 1 or 2 stars in comparability domain AND 2 or 3 stars in outcome/exposure domain.

Fair quality: 2 stars in selection domain AND 1 or 2 stars in comparability domain AND 2 or 3 stars in outcome/exposure domain.

Poor quality: 0 or 1 star in selection domain OR 0 stars in comparability domain OR 0 or 1 stars in outcome/exposure domain.