Table 2.

 Risk of bias in included studies assessed with criteria drawn from Newcastle-Ottawa scale and EPOC group, adapted for assessment of randomised controlled trials, case-control trials, and prospective observational studies according to criteria recommended by Cochrane Drugs and Alcohol Review Group^{28 29}

Study	Selection bias		Performance bias*	Detection bias†	Attrition bias‡	Other bias			Ascertainment bias¶
	Random sequence generation	Allocation concealment				Comparability of cohorts§	Selection of non-exposed	Contamination
Bruneau et al, unpublished	High	High	Low	Low	High	Low	Unclear	Unclear	Low
Chitwood, 199539	High	High	Low	Low	Low	Unclear	Low	Unclear	Low
Deren et al, unpublished	High	High	Low	Low	High	Low	Low	Unclear	Low
Judd et al, unpublished	High	High	Low	Low	High	Low	Unclear	Unclear	Low
Kerr, 200637	High	High	Low	Low	Low	Low	Low	Low	Low
Metzger, 199344	High	High	Low	High	Low	High	Low	Unclear	Low
Moss, 199443	High	High	Low	Low	High	High	High	High	High
Nelson, 20028	High	High	Low	Low	High	High	Low	Unclear	Low
Ruan, 200738	High	High	Low	Low	High	High	Unclear	Unclear	Low
Serpelloni, 199448	High	High	Low	Low	Unclear	Low	Low	Unclear	Unclear
Suntharasamai, 200917	High	High	Low	Low	Low	Low	Low	Low	Low
Van den Berg, 200745	High	High	Low	Low	High	High	Low	Unclear	Low
Vanichseni, 200111	High	High	Low	Low	Unclear	High	Low	Unclear	Low
Williams, 199246	High	High	Low	Unclear	High	High	Low	Unclear	Unclear
Zangerle, 199247	High	High	Low	Low	High	High	Low	Unclear	Low

*Outcome unlikely to be biased by lacking of blinding.

†Objective outcomes, record linkage.

‡Incomplete outcome data.

§Studies deemed at high risk if there was no matching or no adjustment for most important confounding factors.

¶Ascertainment of exposure.