Question of the Week #474

474. A cohort study is done concerning the relationship between exposure to oral contraception and the risk of breast cancer. 1000 women were followed from 2003 to 2007. Results are listed below:

Breast Cancer + Breast Cancer –
Oral contraceptive + 300 (a) 200 (b) a + b = 500

Probability of breast cancer among exposed: 300/500 = 0.6

Oral Contraceptive – 100 © 400 (d) c + d = 500

Probability of breast cancer among those not exposed = 100/500 = 0.2

a + c = 400 b + d = 600

Which of the following represents the absolute risk reduction of breast cancer in women who do not use oral contraceptives?

  1. 75%
  2. 67
  3. 40%
  4. 50%
  5. 30%

2 Responses

  1. 3. or 40%

  2. 2 terms you need to know, control even rate (CER) and experimental even rate(EER). here it is 100/500 for CER= 20%. and EER = 300/500 = 60% which you would expect to be higher as those on OCP have a higher incidence of breast cancer, therefore the absolute risk reduction (ARR) is 60%-20% which is 40%, answer is C

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