Table 1. Bias risk in selected studies.

Type of bias	Case series studies						Case-control studies			Cohort studies
	Koyi et al. 2002	Corner et al. 2005	Barros et al. 2006	Cajoto et al. 2009	Shresthra et al. 2010	Gonzalez- Baracala et al. 2014	Kubik et al. 2002	Hamilton et al. 2005	Iyen-Omofoman et al. 2013	Hoppe et al. 1977	Jones et al. 2007	Hippisley-Cox et al. 2011	Walter et al. 2015
Random sequence generation (selection bias)	$•$	$•$	$•$	$•$	$•$	$•$	$•$	$•$	$•$	$•$	$•$	$•$	$•$
Allocation concealment (selection bias)	$•$	$•$	$•$	$•$	$•$	$•$	$•$	$•$	$•$	$•$	$•$	$•$	$•$
Blinding of participants and personnel (performance bias)	$ₒ$	$ₒ$	$ₒ$	$ₒ$	$ₒ$	$ₒ$	$ₒ$	$ₒ$	$ₒ$	$ₒ$	$ₒ$	$ₒ$	$ₒ$
Blinding outcome assessment (detection bias)	$ₒ$	$ₒ$	$ₒ$	$ₒ$	$ₒ$	$ₒ$	$ₒ$	$ₒ$	$ₒ$	$ₒ$	$ₒ$	$ₒ$	$ₒ$
Incomplete outcome data (attrition bias)	$ₒ$	$ₒ$	$ₒ$	$ₒ$	$ₒ$	$ₒ$	$ₒ$	$ₒ$	$ₒ$	$ₒ$	$ₒ$	$ₒ$	$ₒ$
Selective reporting (attrition bias)	$ₒ$	$ₒ$	$ₒ$	$ₒ$	$ₒ$	$ₒ$	$ₒ$	$ₒ$	$ₒ$	$•$	$•$	$•$	$ₒ$
Other bias	$ₒ$	$ₒ$	$ₒ$	$ₒ$	$ₒ$	$ₒ$	$ₒ$	$ₒ$	$ₒ$	$ₒ$	$ₒ$	$ₒ$	$ₒ$

$•$ Indicates high risk of bias

$•$ Indicates uncertain risk of bias

$ₒ$ Indicates low risk of bias