Suppose that you are interested in determining whether exposure to the organochlorine

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 136 such

differences has mean = 3.1ng/ml and standard deviation ! = 14.1ng/ml.

a. Test the null hypothesis that the mean blood levels of DDE are identical for women

with breast cancer and for healthy control subjects. What do you conclude?

b. Would you expect a 95% confidence interval for the true difference in population

mean DDE levels to contain the value 0? Explain.