Question
A study shows oral contraceptive (OC) use to be associated with an increased risk of stroke. The question arises whether this association could be confounded by age. The data show that oral contraceptive use is inversely associated with age (younger women are more likely to use OCs compared to older women), and that age is positively associated with stroke (older women are more likely to have a stroke compared to younger women).
A. Is age associated with the exposure? Is age associated with the outcome?
B. Calculate the crude OR
Stroke
|
No stroke
|
Total
| |
OC use
|
258
|
2049
|
2307
|
No OC use
|
279
|
2877
|
3156
|
Total
|
537
|
4926
|
5463
|
C. The following table shows the study data stratified by age.
Ages 20-29
|
Ages 30-39
|
Ages 40-49
| ||||
Stroke
|
No stroke
|
Stroke
|
No stroke
|
Stroke
|
No stroke
| |
OC Use
|
39
|
153
|
93
|
903
|
126
|
993
|
No OC Use
|
138
|
855
|
90
|
1389
|
51
|
633
|
Total
|
177
|
1008
|
183
|
2292
|
177
|
1626
|
Calculate stratum-specific odds ratios:
D. Is there evidence that age is an effect modifier in the association between OC use and stroke? Why or why not?
E. Is there evidence that age was a confounder in the association between OC use and stroke? Why or why not?
F. As the investigator, which overall measure of effect (crude or adjusted for age) would you choose to include in your study report?
Source: PH W250 2016 Problem Set 5
