Table 2.

Results of study quality assessment using the Downs and Black assessment tool

Study ID	Reporting	External validity	Internal validity		Power	Total
			Bias	Confounding
Blitzer et al. [34]	9	1	3	3	1	17
Stewart et al. [33]	8	1	2	3	1	15
Brown et al. [32]	9	1	3	3	1	17
Chen et al. [35]	9	1	3	3	1	17
Lee et al. [36]	9	1	3	3	1	17