Here’s a short version of the procedure:

1. V = sum of [(patient value – average value) * coefficient] summed over all the predictors

2.

3.

Some points to keep in mind:

If your software outputs a zero-based baseline survival function, you don’t subtract the average

value from the patient’s value. Instead, calculate the v term as the product of the patient’s predictor

value multiplied by the regression coefficient.

If a predictor is a categorical variable, you have to code the levels as numbers. If you have a

dichotomous variable like pregnancy status, you could code not pregnant = 0 and pregnant = 1.

Then, if in a sample only including women, 47.2 percent of the sample is pregnant, the average

pregnancy status is 0.472. If the patient is not pregnant, the subtraction in Step 1 is 0 – 0.472,

giving –0.472. If the patient is pregnant, you would use the equation 1 – 0.472, giving 0.528. Then

you carry out all the other steps exactly as described.

It’s even a little trickier for multivalued categories (such as different clinical centers) because you

have to code each of these variables as a set of indicator variables.

Estimating the Required Sample Size for a

Survival Regression

Note: Elsewhere in this chapter, we use the word power in its algebraic sense, such as in

is x to the

power of 2. But in this section, we use power in its statistical sense to mean the probability of getting a

statistically significant result when performing a statistical test.

Except for straight-line regression discussed in Chapter 16, sample-size calculations for regression

analysis tend not to be straightforward. If you find software that will calculate sample-size estimates

for survival regression, it often asks for inputs you don’t have.

Very often, sample-size estimates for studies that use regression methods are based on simpler

analytical methods. We recommend that when you’re planning a study that will be analyzed using

PH regression, you base your sample-size estimate on the simpler log-rank test, described in

Chapter 22. The free PS program handles these calculations very well.

You still have to specify the following: