Question 1: In a case-control study, 101 stroke patients were compared with 137
ID: 3059693 • Letter: Q
Question
Question 1:
In a case-control study, 101 stroke patients were compared with 137 healthy controls. Among the results were: Cigarette smoking by stroke patients and healthy controls
Smoker
Non smoker
Total
Case
71
30
101
Controls
36
101
137
Total
107
131
238
(a) Find the increase in odds for having a stroke among smokers compared to nonsmokers (ie find the odds ratio OR).
(b) Find the increase in risk for having a stroke among smokers compared to nonsmokers (ie find the risk ratio RR).
(c) Which measure of association should be reported, OR or RR?
(d) When the incidence is rare, the OR from a case-control study approximates the RR that would be obtained from a cohort study. To see this, suppose the incidence of stroke is 7 per 1,000. We can approximate this incidence by simply multiplying the controls row of the table by 100. Note that this does not change the value of the OR. Calculate the RR for this table.
Smoker
Non smoker
Total
Case
71
30
101
Controls
3600
10100
13700
Total
3671
10130
13801
Smoker
Non smoker
Total
Case
71
30
101
Controls
36
101
137
Total
107
131
238
Explanation / Answer
Result:
Question 1:
In a case-control study, 101 stroke patients were compared with 137 healthy controls. Among the results were: Cigarette smoking by stroke patients and healthy controls
Smoker
Non smoker
Total
Case
71
30
101
Controls
36
101
137
Total
107
131
238
(a) Find the increase in odds for having a stroke among smokers compared to nonsmokers (ie find the odds ratio OR).
Odds ratio = (71*101)/(36*30) =6.6398
(b) Find the increase in risk for having a stroke among smokers compared to nonsmokers (ie find the risk ratio RR).
RR = (71/107)/(30/131) =2.8975
(c) Which measure of association should be reported, OR or RR?
Since this is a case control study, we should report OR.
(d) When the incidence is rare, the OR from a case-control study approximates the RR that would be obtained from a cohort study. To see this, suppose the incidence of stroke is 7 per 1,000. We can approximate this incidence by simply multiplying the controls row of the table by 100. Note that this does not change the value of the OR. Calculate the RR for this table.
Smoker
Non smoker
Total
Case
71
30
101
Controls
3600
10100
13700
Total
3671
10130
13801
Odds ratio = (71*10100)/(3600*30) =6.6398
RR = (71/3671)/(30/10130) =6.5307
Smoker
Non smoker
Total
Case
71
30
101
Controls
36
101
137
Total
107
131
238
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