Hazard ratio: when risk changes over the course of time

Survival variables are used very frequently in clinical trials when the objective is measuring not only the happening of the event itself but also the moment it happens. In these cases, the measure of relative risk between intervention and control groups would produce an incomplete information.

In this paper the use and meaning of instantaneous risk rate, better known by the name of hazard ratio, is described. This statistic takes into account the time effect, so that it is appropriate for the assessment of risk when there is a survival variable involved.