Q1. Name the other common type of bias which might take place in this type of study. Is it likely to have affected the results here? Describe your reasoning in brief.

Q2. What potential confounders encompass the authors recognized? How have they dealt with them at the design phase and throughout analysis? Is there likely to be the residual confounding?

Q3. Do the authors describe the role of chance in this study? Describe this in relation to the outcomes and what effect might it encompass on the internal validity.

Q4. In describing their findings, the authors have ignored a main threat to the validity of their estimates of exposure. describe and describe this error.

[Note: it is usually a concern only when studying exposures of this kind].

What alternative study design must overcome this problem and describe why?

Q5. Rate the study’s internal validity; low, medium and high. Describe this systematically in relation to the data and other evidence.

To whom might the results be generalized? Describe your answer in brief.