Introduction to Stata 8

13. Survival analysis and related issues
st [ST] manual
The st family of commands includes a number of facilities, described in the Survival
Analysis manual [ST]. Here I describe the stset and stsplit commands and give a few
examples. The data is cancer1.dta, a modification of the cancer.dta sample data
accompanying Stata.
The observation starts at randomization (agein), the data set includes these variables:
. summvl // summvl is a summarize displaying variable labels.
// Get it by: findit summvl
Variable Obs Mean Std.Dev Min Max Label
------------------------------------------------------------------------
lbnr 48 24.5 14 1 48 Patient ID
drug 48 1.875 .841099 1 3 Drug type (1=placebo)
drug01 48 .583333 .498224 0 1 Drug: placebo or active
agein 48 56.398 5.6763 47.0955 67.8746 Age at randomization
ageout 48 57.6896 5.45418 49.0122 68.8737 Age at death or cens.
risktime 48 1.29167 .854691 .083333 3.25 Years to death or cens.
died 48 .645833 .483321 0 1 1 if patient died
stset [ST] stset
stset declares the data in memory to be survival time (st) data. I create two versions: In
st.cancer1.dta time simply is risktime, age not taken into consideration. In
st.cancer2.dta time at risk is defined by age at entry (agein) and exit (ageout)
enabling to study and control for the effect of age.
Simple analysis – age not included
stset data with risktime as the time-of-exit variable:
. // c:\dokumenter\proj1\gen.st.cancer1.do
. use c:\dokumenter\proj1\cancer1.dta , clear
.
stset risktime , failure(died==1) id(lbnr)
id: lbnr
failure event: died == 1
obs. time interval: (risktime[_n-1], risktime]
exit on or before: failure
-------------------------------------------------------------------------
48 total obs.
0 exclusions
-------------------------------------------------------------------------
48 obs. remaining, representing
48 subjects
31 failures in single failure-per-subject data
62 total analysis time at risk, at risk from t = 0
earliest observed entry t = 0
last observed exit t = 3.25
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