338 PART 6 Analyzing Survival Data

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» ^{The treatment group variable. In this case, we created the variable}

Radiation, and coded it as 1 if the participant was in the radiation group,

and 0 if they were in the chemotherapy group. That way, the coefficient

produced in the output will indicate the increase or decrease in proportional

hazard associated with being in the radiation group compared to the chemo-

therapy group.»

» ^{The clinical center variable. In this case, we choose to create an indicator}

variable called CenterCD, which is 1 if the participant is from Center C or

Center D, and is 0 if they are from A or B. Alternatively, you could choose to

create one indicator variable for each center, as described in Chapter 18.

If you use a numerical variable such as age as a predictor and enter it into the

model, the resulting coefficient will apply to increasing this variable by one unit

(such as for one year of age).

Using the R statistical software, the PH regression can be invoked with a single

command:

coxph(formula = Surv(Time, Status) ~ CenterCD + Radiation)

Figure 23-4 shows R’s output, using the data that we graph in Figure 23-3. The

output from other statistical programs won’t look exactly like Figure 23-4, but

you should be able to find the main components described in the following

sections.

FIGURE 23-4:

Output of a PH

regression

from R.