Ct4 PH Cox Model

Question 1

Write the equation of the cox model. Define all the terms you use.

Answer 1

Answer in 2005 B1 Exam

Question 2

Explain why the cox model is sometimes described as “semi-parametric”

Answer 2

The model is semi-parametric because it is possible to estimate beta from the data without estimating the baseline hazard.

Therefore the baseline can have any shap determined by the data.

Question 3

Explain why the Gompertz hazard might be appropriate for estimating the mortality of live aged 50 years to-
65 years

Answer 3

Taking logarithims of the Gompertz hazard produces
log(lamdax)= log(beta) + xlogC
which indicates that the rate of increase of the hazard with age is constant. Empirically, this is often a reasonable assumption for the middle ages and older ages, which include the range 50 -65 years.