Most medical studies, including this one, summarize differences in risks by estimating a hazard ratio. A hazard function gives the conditional probability densi

Most medical studies, including this one, summarize differences in risks by estimating a hazard ratio. A hazard function gives the conditional probability density of having an event at time T, given that no event has occurred at time t < T. (E.g. the probability that a hard drive first fails in its 14937th hour of operation, except that formally speaking we're talking about the probability density of failing not in an hour, but an infinitessimal instant.) A hazard ratio is the ratio of two hazard functions. Just as with hard drives, death is certain (eventually) but we can still prefer a hard drive with MTTF of 5 years vs. one with MTTF of 3 years.