Table 5. Assessment of the studies included in the systematic review according to the appraisal tool for cross-sectional studies (AXIS).

Questions	Yes	No	Uncertain	Not applicable
Introduction
Were the aims/objectives of the study clear?	27	0	0	0
Methods
Was the study design appropriate for the stated aim(s)?	11	16	0	0
Was the sample size justified?	0	27	0	0
Was the target/reference population clearly defined?	19	8	0	0
Was the sample frame taken from an appropriate population base so that it closely represented the target/reference population under investigation?	19	0	8	0
Was the selection process likely to select subjects/participants that were representative of the target/reference population under investigation?	6	16	0	5
Were measures undertaken to address and categorise non-responders?	2	2	0	23
Were the risk factor and outcome variables measured appropriate to the aims of the study?	27	0	0	0
Were the risk factor and outcome variables measured correctly using instruments/measurements that had been trialled, piloted or published previously?	10	17	0	0
Is it clear what was used to determine statistical significance and/or precision estimates?	16	5	0	6
Were the methods (including statistical methods) sufficiently described to enable them to be repeated?	20	7	0	0
Results
Were the basic data adequately described?	26	1	0	0
Does the response rate raise concerns about non-response bias?	0	27	0	0
If appropriate, was information about non-responders described?	2	2	0	23
Were the results internally consistent?	1	25	0	1
Were the results presented for all the analyses described in the methods?	22	5	0	0
Discussion
Were the authors’ discussions and conclusions justified by the results?	27	0	0	0
Were the limitations of the study discussed?	18	9	0	0
Other
Were there any funding sources or conflicts of interest that may affect the authors’ interpretation of the results?	0	9	18	0
Was ethical approval or consent of participants attained?	14	0	13	0