Table 2. Quality assessment of OCSs by NOS scale.

Study	Items	Wästerlid 2017 (20)	Knauf 2019 (19)
Selection	Representativeness of the exposed cohort	*	*
	Selection of the non-exposed cohort	*	*
	Ascertainment of exposure	*	*
	Demonstration that outcome of interest was not present at start of study	*	*
Comparability	Comparability of cohorts on the basis of the design or analysis	**	**
Outcome	Assessment of outcome	*	*
	Was follow-up long enough for outcomes to occur	*	*
	Adequacy of follow up of cohorts	*	*

*, star-rating in NOS, each study can have a maximum of one star per entry in “Selection”, “Outcome” and a maximum of two stars per entry in “Comparability”. OCSs, observational comparative studies; NOS, Newcastle-Ottawa scale.