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Table 1 —

Assessment of internal validity based on Cochrane Collaboration Criteria (20)

Type of biasDefinition
Selection bias Systematic differences in control and intervention groups at baseline To avoid requires randomization and no significant differences between baseline variables in control and intervention groups, or adequate statistical consideration of potential confounders if baseline differences exist 
Performance bias Systematic differences in care provided to control and intervention groups, apart from the intervention being evaluated To avoid requires no evidence of contamination or cointervention, including no additional contacts with researcher or providers for the intervention group compared with the control group 
Attrition bias Systematic differences between study groups in withdrawals from the study To avoid requires attrition <20% of total n, or dropouts must resemble completers in baseline characteristics 
Detection bias Systematic differences in outcomes assessment between study groups To avoid requires blinding for any outcome subject to assessor interpretation 
Type of biasDefinition
Selection bias Systematic differences in control and intervention groups at baseline To avoid requires randomization and no significant differences between baseline variables in control and intervention groups, or adequate statistical consideration of potential confounders if baseline differences exist 
Performance bias Systematic differences in care provided to control and intervention groups, apart from the intervention being evaluated To avoid requires no evidence of contamination or cointervention, including no additional contacts with researcher or providers for the intervention group compared with the control group 
Attrition bias Systematic differences between study groups in withdrawals from the study To avoid requires attrition <20% of total n, or dropouts must resemble completers in baseline characteristics 
Detection bias Systematic differences in outcomes assessment between study groups To avoid requires blinding for any outcome subject to assessor interpretation 
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