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Unformatted text preview: men at age forty who participate in routine screening have breast cancer. 80% of women
with breast cancer will get positive mammographies. 9.6% of women without breast cancer will also
get positive mammographies. A woman in this age group had a positive mammography in a routine
screening. What is the probability that she actually has breast cancer?
What do we know?  P (cancer) = 0.01, P (test pos.  cancer) = 0.8, and P (test pos.  no cancer) =
0.096.
We want to ﬁnd P (cancer  test pos.)
This is an application of Bayes theorem:
P (cancer  test pos.) =
=
= 3 P (cancer...
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 Spring '08
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