where HYPERT and UIRRIT are hypertension and uterine irritability. The
default values for SLENTRY (pvalue to enter) and SLSTAY (pvalue to stay)
are 0.05 and 0.10, respectively.
EXERCISES
11.1 Given the small data set9 ; 13 ; 13 þ; 18 ; 23 ; 28 þ; 31 ; 34 ; 45 þ; 48 ; 161 þcalculate and graph the Kaplan–Meier curve.
11.2 A group of 12 hemophiliacs, all under 41 years of age at the time of
HIV seroconversion, were followed from primary AIDS diagnosis until
death (ideally, we should take as a starting point the time at which a
person contracts AIDS rather than the time at which the patient is
diagnosed, but this information is not available). Survival times (in
months) from diagnosis until death of these hemophiliacs were: 2, 3, 6,
6, 7, 10, 15, 15, 16, 27, 30, and 32. Calculate and graph the Kaplan–
Meier curve.
11.3 Suppose that we are interested in studying patients with systemic cancer
who subsequently develop a brain metastasis; our ultimate goal is to
prolong their lives by controlling the disease. A sample of 23 such
patients, all of whom were treated with radiotherapy, were followed
from the first day of their treatment until recurrence of the original
tumor. Recurrence is defined as the reappearance of a metastasis in
exactly the same site, or in the case of patients whose tumor never
completely disappeared, enlargement of the original lesion. Times to
recurrence (in weeks) for the 23 patients were: 2, 2, 2, 3, 4, 5, 5, 6, 7, 8,
9, 10, 14, 14, 18, 19, 20, 22, 22, 31, 33, 39, and 195. Calculate and
graph the Kaplan–Meier curve.
11.4 A laboratory investigator interested in the relationship between diet
and the development of tumors divided 90 rats into three groups and
fed them low-fat, saturated-fat, and unsaturated-fat diets, respectively.
The rats were of the same age and species and were in similar physical
condition. An identical amount of tumor cells was injected into a foot
pad of each rat. The tumor-free time is the time from injection of tumor
cells to the time that a tumor develops; all 30 rats in the unsaturated-fat
diet group developed tumors; tumor-free times (in days) were: 112, 68,
84, 109, 153, 143, 60, 70, 98, 164, 63, 63, 77, 91, 91, 66, 70, 77, 63, 66,
66, 94, 101, 105, 108, 112, 115, 126, 161, and 178. Calculate and graph
the Kaplan–Meier curve.426 ANALYSIS OF SURVIVAL DATA