1 1 pts question 3 9152018 quiz 2 dags case control

1 / 1 pts
Question 3

9/15/2018 Quiz 2 – DAGs / Case-control design: EPI 201: Introduction to Epidemiology: Methods I
4/10
Investigators conducted an observational study of past vitamin E supplementation and infertility. The
researchers evaluated the relationship between taking a daily vitamin E supplement over the past year and
abnormal sperm count among males. The participants were all recruited from a health clinic. Health­
conscious men who took vitamin supplements were more likely to visit the health clinic compared to men
who did not take vitamin supplements. Additionally, men having difficulty conceiving were also more likely to
attend 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 1
b. DAG 2
c. DAG 3
Correct!
Correct!

9/15/2018 Quiz 2 – DAGs / Case-control design: EPI 201: Introduction to Epidemiology: Methods I
5/10
The correct answer is C. We know that men who took vitamin supplements were more likely to visit
the health clinic compared to men who did not take vitamin supplements; thus there should be an
arrow going from ‘vitamin E supplementation’ to ‘visiting a health clinic’. We also know that men
having difficulty conceiving were more likely to attend the health clinic; thus there should also be an
arrow going from ‘sperm count’ to ‘visiting a health clinic’. There is a box around ‘visiting a health
clinic’, because we are restricting our analysis to men who were observed in the health clinic. This is
represented in DAG 3. In DAG 1, like in DAG 3, there is an arrow going from ‘vitamin E
supplementation’ 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 not
supported by the information given above; we have been told that visiting a health clinic is affected by
sperm count instead of the other way round, and hence choice A is incorrect. In DAG 2, there are
arrows 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 a
health clinic is affected by sperm count and vitamin E supplementation, not the other way round;
hence choice B is incorrect.
1 / 1 pts