Suppose that you are interested in determining whether exposure to the organo-chlorine DDT, which has been used extensively as an insecticide for many years, is associated with breast cancer in women. As part of a study that investigated this issue, blood was drawn from a sample of women diagnosed with breast cancer over a six-year period and from a sample of healthy control subjects matched to the cancer patients on age, menopausal status, and date of blood donation. Each woman's blood level of DDE - an important byproduct of DDT in the human body - was measured, and the difference in levels for each patient and her matched control calculated. A sample of 171 such differences has mean d ¯=2.7 ng/ml and standard deviation s_d=15.9 ng/ml.