Screening Mammography Screening mammography for breast cancer has an overall likelihood ratio of about 9 for a positive test: The odds of breast cancer are about nine times higher in a woman with a...

Screening Mammography

Screening mammography for breast cancer has an overall likelihood ratio of about 9 for a positive test: The odds of breast cancer are about nine times higher in a woman with a positive mammogram than a woman with a negative mammogram (Breast Cancer Surveillance Consortium, 2013). How likely is it that an average adult woman who gets a positive mammogram actually has breast cancer? What if she is younger than 40 years of age? A Bayesian reasoner, considering a pretest breast cancer probability of 0.1% (1 breast cancer per 1000 women, or 1 : 999 odds), should conclude that a woman with a positive mammogram will have about a 1% posttest probability of breast cancer. In a woman younger than 40 years of age, however, not only is the pretest probability lower than average (closer to .005%, or 5 breast cancers per 100,000 women), but the likelihood ratio of a positive test is lower (closer to 5). As a result, a positive test in a young woman suggests a 0.02% probability of breast cancer. How accurate was your estimate of the posttest probability, the pretest probability and the strength of the test?