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1 / 1 ptsQuestion 3
9/15/2018Quiz 2 – DAGs / Case-control design: EPI 201: Introduction to Epidemiology: Methods I4/10Investigators conducted an observational study of past vitamin E supplementation and infertility. Theresearchers evaluated the relationship between taking a daily vitamin E supplement over the past year andabnormal sperm count among males. The participants were all recruited from a health clinic. Healthconscious men who took vitamin supplements were more likely to visit the health clinic compared to menwho did not take vitamin supplements. Additionally, men having difficulty conceiving were also more likely toattend the health clinic. Which of the DAGs above represents the study the investigators conducted?(assume the assumptions of welldefined interventions and positivity are met)a. DAG 1b. DAG 2c. DAG 3Correct!Correct!
9/15/2018Quiz 2 – DAGs / Case-control design: EPI 201: Introduction to Epidemiology: Methods I5/10The correct answer is C. We know that men who took vitamin supplements were more likely to visitthe health clinic compared to men who did not take vitamin supplements; thus there should be anarrow going from ‘vitamin E supplementation’ to ‘visiting a health clinic’. We also know that menhaving difficulty conceiving were more likely to attend the health clinic; thus there should also be anarrow going from ‘sperm count’ to ‘visiting a health clinic’. There is a box around ‘visiting a healthclinic’, because we are restricting our analysis to men who were observed in the health clinic. This isrepresented in DAG 3. In DAG 1, like in DAG 3, there is an arrow going from ‘vitamin Esupplementation’ to ‘visiting a health clinic’, which is correct based on the information provided.However, there is also an arrow going from ‘visiting a health clinic’ to ‘sperm count’, which is notsupported by the information given above; we have been told that visiting a health clinic is affected bysperm count instead of the other way round, and hence choice A is incorrect. In DAG 2, there arearrows going from ‘visiting a health clinic’ to both ‘vitamin E supplementation’ and ‘sperm count’,which again is not supported by the information provided above. The probability that a man will visit ahealth clinic is affected by sperm count and vitamin E supplementation, not the other way round;hence choice B is incorrect.1 / 1 pts