the nodes and removing tissue samples to examine under the microscope
for evidence of cancer. However, certain variables that can be measured
without surgery are predictive of the nodal involvement. The purpose of
the study presented here was to examine the data for 53 prostate cancer
patients receiving surgery, to determine which of five preoperative vari-
ables are predictive of nodal involvement. Table E7.23 presents the
complete data set. For each of the 53 patients, there are two continuous
independent variables: age at diagnosis and level of serum acid phos-
phatase (100; called ‘‘acid’’); and three binary variables: x-ray reading,
pathology reading (grade) of a biopsy of the tumor obtained by needle
before surgery, and a rough measure of the size and location of the
tumor (stage) obtained by palpation with the fingers via the rectum. In
addition, the sixth column presents the findings at surgery—the primary
outcome of interest, which is binary, a value of 1 denoting nodal
involvement, and a value of 0 denoting no nodal involvement found at
surgery. The three binary factors have been investigated previously; this
exercise is focused on the e¤ects of the two continuous factors (age and
acid phosphatase). Test to compare the group with nodal involvement
and the group without, using:
(a) The two-samplettest.
(b)Wilcoxon’s rank-sum test.
EXERCISES 281