Table 1 Quality rating assessment criteria*

Clearly specified and appropriate research question

Clear details and justification of study design, including selection of cases and controls

Detailed description of research setting, data collection methods and type of analysis performed

Logical presentation and discussion of results and study conclusions

Adequate sample size and response rate (>60%) relative to study design

Overall ratings

Assessment criteria

+++

All of the above criteria fulfilled

++

Almost all of the above criteria fulfilled, and those criteria that were not fulfilled were thought unlikely to alter the conclusions of the study

+

Some of the criteria were fulfilled, and those criteria that were not fulfilled were thought unlikely to alter the conclusions of the study