Friday, May 25, 2012

Hazards ratio, Relative risk

HR = Probability of event in treatment grp/Probability of event in control group

RR = Probability of event in treatment grp/Probability of event in control group

Exactly! same formular!

Hazard ratio is actually a type of Relative risk (a.k.a Risk ratio)

The difference?
HR is instantaneous while RR is culmulative.
RR can only be calculated at the end of the study while HR can be calculated at different points in time. Hazard ratios are calculated using the survival analysis technique. This keeps track of the number of subjects who haven't had the event (e.g death) at a particular time.

Again, HR is an estimate of RR

HR of 1 means the rate of occurrence of the event in treatment/intervention grp is same as in control/placebo grp

HR >1  means event occurs more in the intervention grp

HR <1  means event occurs less in the intervention grp

To convert a HR to probability,

Probability of event occurring = HR/1+ HR

So, HR of 1 is a probability of 50%. A 50:50 chance, i.e treatment and placebo grps are the same in terms of outcome



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