Table 2 Bias assessment for the cross-sectional studies identified through the systematic search. Risk of bias assessment with 8 check-list items for each individual study

Author

Were the criteria for inclusion in the sample clearly defined?

Were the study subjects and the setting described in detail?

Was the exposure measured in a valid and reliable way?

Were objective, standard criteria used for measurement of the condition?

Were confounding factors identified?

Were strategies to deal with confounding factors stated?

Were the outcomes measured in a valid and reliable way?

Was appropriate statistical analysis used?