Table 3

Quality assessment of studies included in the systematic review

StudySelection biasExposure biasOutcome assessment biasConfounding factor biasAnalytical biasAttrition biasOverall likelihood of bias
Black et al23LowMinimalMinimalLowMinimalModerateLow
Gendelman et al24ModerateMinimalMinimalModerateModerateLowModerate
Lin et al25MinimalMinimalMinimalMinimalMinimalLowMinimal
Pearson et al26LowLowMinimalModerateModerateModerateModerate
Salmi et al27MinimalLowLowNot reportedNot reportedMinimalModerate
Williams et al13LowLowLowMinimalLowLowLow
Winkley et al28LowMinimalMinimalMinimalLowMinimalMinimal
  • The six different types of bias were classified as ‘minimal’, ‘low’, ‘moderate’, ‘high’ or ‘not reported’ based on criteria outlined previously in O’Neill et al.22