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Table 3. Power to detect differences in risk due to environmental effects in the prospective study as a function of follow-up time, exposure frequency and risk ratio of the exposure. The total number of cancer cases is assumed to be those given in table 1 above assuming 2000 females enrolled, 2/3 of which are unaffected at time of enrollment, a 30% rate of prophylactic mastectomy and a 50% rate of oophorectomy.

From: The International BRCA1 /2Carrier Cohort Study: Purpose, Rationale, and Study Design

   Breast Cancer Ovarian Cancer
Risk Ratio Exposure Frequency 5 yrs 10 yrs 5 yrs 10 yrs
2 0.1 0.64 0.87 Not done Not done
  0.3 0.80 0.98 Not done Not done
3 0.1 0.95 1.00 0.48 0.68
  0.3 0.98 1.00 0.56 0.82
5 0.1 1.00 1.00 0.76 0.93
  0.3 1.00 1.00 0.81 0.97