7. An investigator was interested in the association between smoking and breast
ID: 138269 • Letter: 7
Question
7. An investigator was interested in the association between smoking and breast cancer. She randomly selected 10,000 individuals from community C and followed them for 10 years. At baseline, 10% of the 10,000 subjects were smokers and 25% of these smokers already had breast cancer. During the 10 years of follow-up, 400 additional smokers developed breast cancer, and 200 subjects who did not smoke developed breast cancer. Fill in the 2x2 table for the entire community sample at baseline. Assume no prevalent cases of breast cancer in non-smokers at baseline a. b. Fill in the 2x2 table for the population at the end of follow-up. c. Calculate the cumulative incidence of breast cancer among non-smokers. d. Calculate the cumulative incidence of breast cancer among smokers. EPI 100 e. Calculate the relative risk and interpret your result.Explanation / Answer
a. 2x2 table for the community at baseline
Smokers
Non Smokers
Breast Cancer
250
-
No Breast Cancer
750
9000
b. 2x2 table for the community after follow up
Smokers
Non Smokers
Breast Cancer
650
200
No Breast Cancer
350
8800
c. Cumulative incidence of breast cancer among non smokers
Smokers
Non Smokers
TOTAL
Breast Cancer
650
200
850
No Breast Cancer
350
8800
9150
TOTAL
1000
9000
10000
Cumulative Incidence = Number of new cases observed/ Number of subjects at risk (At the beginning of the study)
Cumulative incidence for among non smokers = 200/9000 = 0.0222 =2.22%
d. Cumulative Incidence = Number of new cases observed/ Number of subjects at risk (At the beginning of the study)
Cumulative incidence for among smokers = 400/750 = 0.5333 = 53.33%
e. Relative risk
Smokers
Non Smokers
TOTAL
Breast Cancer
650 (a)
200 (b)
850 (a+b)
No Breast Cancer
350 (c)
8800 (d)
9150 (c+d)
TOTAL
1000
9000
10000
Relative risk = a/ (a+b)
c/(c+d)
= 650/850 = 20.10
350/9150
Since the relative risk is more than 1, it is interpreted that smokers are 20 times as likely as non -smokers to get breast cancer.
Smokers
Non Smokers
Breast Cancer
250
-
No Breast Cancer
750
9000
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