Table 2. Summary of quality assessment using the Newcastle-Ottawa Scale for longitudinal studies.

Longitudinal studies
		Selection				Comparability	Outcome
No	Author	Representativeness of exposed cohort	Selection of the non- exposed cohort	Ascertainment of exposure	Demonstration that outcome of interest was not	Comparability of cohorts on the basis of the design or analysis	Assessment of outcome	Was follow- up long enough for	Adequacy of follow up of cohort	Total
9	Chen et al. (2019)	*	*	*	*	**		*	*	8
15	González-Castro et al. (2020)	*	*	*	*	**		*		7
16	Hatch et al. (2016)	*	*	*		**		*		6
24	Ronda-Pérez et al. (2019)	*	*	*		**		*		6

Good quality: 3 or 4 stars in the 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.