A paper describes a study of the use of MRI (Magnetic Resonance Imaging) exams in the diagnosis of breast cancer. The purpose of the study was to determine if MRI exams do a better job than mammograms...

A paper describes a study of the use of MRI (Magnetic Resonance Imaging) exams in the diagnosis of breast cancer. The purpose of the study was to determine if MRI exams do a better job than mammograms of determining if women who have recently been diagnosed with cancer in one breast have cancer in the other breast. The study participants were 940 women who had been diagnosed with cancer in one breast and for whom a mammogram did not detect cancer in the other breast.


These women had an MRI exam of the other breast, and 111 of those exams indicated possible cancer. After undergoing biopsies, it was determined that 30 of the 111 did in fact have cancer in the other breast, whereas 81 did not. The women were all followed for one year, and four of the women for whom the MRI exam did not indicate cancer in the other breast were subsequently diagnosed with cancer that the MRI did not detect. The accompanying table summarizes this information.

Suppose that for women recently diagnosed with cancer in only one breast, the MRI is used to decide between the two "hypotheses."

(Although these are not hypotheses about a population characteristic, this exercise illustrates the definitions of Type I and Type II errors.)