1) List another common type of bias that can takes place in this type of study. Is it likely to have influenced the results here? Explain your reasoning briefly.

2) What potential confounders have the authors recognised? How have they dealt with them at the design stage and during analysis? Is there likely to be residual confounding?

3) Whether the authors discuss the role of chance in this study? Describe this in relation to the results and what effect might it have on the internal validity.