Table 1

Risk of bias and quality assessment criteria for prevalence studies

Item under review	Quality score (points)
External validity
Was the study's target population a close representation of the national population in relation to relevant variables?	1
Was the sampling frame a true or close representation of the target population?	1
Was some form of random selection used to select the sample, OR was a census undertaken?	1
Was the likelihood of non-response bias minimal?	1
Total	4 points
Internal validity
Were data collected directly from the participants (as opposed to a proxy)?	1
Was an acceptable case definition used in the study?	1
Was the study instrument that measured the parameter of interest shown to have validity and reliability?	1
Was the same mode of data collection used for all subjects?	1
Was the length of the shortest prevalence period for the parameter of interest appropriate?	1
Were the numerator(s) and denominator(s) for the parameter of interest appropriate?	1
Total	6 points
Summary item on the overall risk of study bias (low, moderate or high)

As described by Hoy et al, the summary assessment evaluates the overall risk of study bias and is based on the rater's subjective judgement given responses to the preceding 10 items. This approach is consistent with the Cochrane and GRADE working group24 recommendation or approaches. Furthermore, as summarised in the PRISMA elaboration document, summative scales that numerically summarise multiple components into a single number are misleading and unhelpful,25 hence our choice of an overall ordinal scale for risk of bias. Response options for individual items are either low (1) or high risk of bias (0). If there is insufficient information in the article to permit judgement of a particular item, then the article is deemed to be at high risk of bias with respect to that item.19 ,26 ,27
GRADE, Grading of Recommendation, Assessment, Development and Evaluation; PRISMA, The Preferred Reporting Items for Systematic reviews and Meta-Analyses.