Table 4

Risk of bias of observational studies

Similar populationAssessment of exposureOutcome not present at start of studyAdjustment of prognostic variablesAssessment of prognostic variablesAssessment of outcomeAdequate follow-up
von Knorring et al36+++−−−−+
Chuang et al38+++−−−−++−−+
Godeau et al37−−−−−−−−−−
Behn et al39++++−−−−−−−−+
Gouet et al40++−−−−−−−−+
Andersson et al41++++−−−−−−−−+
Delecoeuillerie et al42++++−−−−−−−−+
Nesher et al43++++−−+−−+
Gabriel et al44+++++++++++
Proven et al45+++++++++++
Hutchings et al46++++−−+++−−
Schmidt et al49++++++−−
Dasgupta et al50+−−−−−−+
Khalifa et al51++−−−−−−−−−−
Martinez-Lado et al52+++−−+++−−+
Mazzantini et al53+++++++++++
Dunstan et al54++++++++−−+
Alba et al55+++++++−−++
Carbonella et al58+−−−−−−−−−−
Farschou et al31+++++++++++
Muller et al57+++−−−−+−−
  • Question 1: was selection of exposed and non-exposed cohorts drawn from the same population? Question 2: can we be confident in the assessment of exposure? Question 3: can we be confident that the outcome of interest was not present at start of study? Question 4: did the study match exposed and unexposed for all variables that are associated with outcome of interest or did the statistical analysis adjust for these prognostic variables? Question 5: can we be confident in the assessment of the presence or absence of prognostic factors? Question 6: can we be confident in the assessment of outcome? Question 7: was the follow up of cohorts adequate?

  • ++, definitely yes (low risk of bias); +, probably yes; −, probably no; −−, definitely no (high risk of bias).