Table 2.

Study quality assessment for each study.

Quality assessment items	Study and quality assessment scorea
	Aldiss [57]	Burbank [39]	Cafazzo [55]	Frøisland [56]
Selection bias	Weak	Weak	Weak	Weak
Study design	Moderate	Weak	Moderate	Weak
Confounders	Weak	Weak	Weak	Weak
Blinding	Moderate	Weak	Weak	Weak
Data collection method	Moderate	Strong	Strong	Weak
Withdrawals and dropouts	Moderate	Strong	Moderate	Strong

^aItems were scored using the Effective Public Health Practice Project (EPHPP) quality assessment tool.