Table 2.

Risk of bias according to the modified Newcastle-Ottawa Scale for cross-sectional studies [23]

Item	Oen [9]	Seale and Glickman 2007 [29]	Weber [8]	McBride [35]	Stangvaltaite [7]	Schwendicke [28]	Chisini [34]	Schwendicke [6]	Katz [27]
Selection
Representativeness of the sample	*		*	*	*	*	*	*	*
Sample size determination
Non-responders				*		*
Validity of survey
Checked reliability and internal consistency Survey available				**		**	*	**
Comparability	*	–	–	–	*	*	*	*	–
Outcome
Assessment of the outcome	*	*	*	*	*	*	*	*	*
Statistical test			*		*	*		*
Overall	***	*	***	*****	****	*******	****	******	**

For each risk of bias domain, one to two stars could be collected, with a total number of eight stars being possible. We classified studies as high risk (1–3 stars), moderate risk (4–6 stars), or low risk (7–8 stars)