Hazard ratio (HR) is a measure of an effect of an intervention on an outcome of interest over time
- reported most commonly in time-to-event analysis or survival analysis (i.e. when we are interested in knowing how long it takes for a particular event/outcome to occur)
- outcome could be an adverse/negative outcome (e.g. time from treatment/surgery until death/relapse) or a positive outcome (e.g. time to cure/discharge/conceive/heal or disease-free survival).
Hazard Ratio (i.e. the ratio of hazards) = Hazard in the intervention group ÷ Hazard in the control group
- hazard represents the instantaneous event rate
- means the probability that an individual would experience an event (e.g. death/relapse) at a particular given point in time after the intervention, assuming that this individual has survived to that particular point of time without experiencing any event
Confidence Interval (CI):
- the range of values that is likely to include the true population value and is used to measure the precision of the study’s estimate (in this case, the precision of the Hazard Ratio)
- narrower the confidence interval, the more precise the estimate
- if the confidence interval includes 1, then the hazard ratio is not significant.
Interpretation of Hazard Ratio
- Because Hazard Ratio is a ratio, then when:
- HR = 0.5: at any particular time, half as many patients in the treatment group are experiencing an event compared to the control group
- HR = 1: at any particular time, event rates are the same in both groups
- HR = 2: at any particular time, twice as many patients in the treatment group are experiencing an event compared to the control group
Hazard Ratios vs. Risk Ratios (or Relative Risk)
- Hazard ratio is frequently interpreted as risk ratio (or relative risk), but they are not technically the same
- one of the main differences between risk ratio and hazard ratio is that risk ratio does not care about the timing of the event but only about the occurrence of the event by the end of the study (i.e. whether they occurred or not: the total number of events by the end of the study period)
- in contrast, hazard ratio takes account not only of the total number of events, but also of the timing of each event.
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