The Cox Proportional Hazards Model
It is important to note that although death rates are relative frequencies, they do not correspond to those relative frequencies we see usually in a histogram describing the distribution of a continuous covariate. Indeed, the number of subjects dying in the fifth year is much smaller than the number of subjects dying in the first year, although the death rates are similar. This is due to the fact that death rates correspond to conditional probabilities. They describe the probability of a patient to die in a certain interval, given the patient is alive at the start of the interval.