1) State another common type of the bias which may takes place in this type of study. Whether it is likely to have influenced the results here? Describe your reasoning in brief.

2) Specify the potential confounders that have been recognised by the authors? Explain how have they dealt with them at the design stage and at the time of analysis? Whether there is likely to be the residual confounding?

3) State whether the authors discuss the role of chance within this study? Explain this in relation to results and what are the effect might it have on the internal validity.