Absolute risk reduction is just the risk of an adverse outcome without treatment minus the risk of an adverse outcome with treatment. Both statistics could be used to aid patients and practitioners to balance the reduction in risk with regard to an adverse event against the cost and negative effects of the proposed treatment.
When a clinician prescribes a drug to deal with an illness, the goal is to either cure the disease or to lessen the likelihood that a second effect of the illness will take place. For instance, when a septic patient is treated with an antibiotic, the goal is always to cure the patient. A different way to view that goal is that the purpose of the antibiotic is to decrease the patient's risk of passing away from the infection.
Calculating absolute risk reduction or absolute risk increase for any treatment intervention is easy. It is simply the difference between the proportion of subjects with the result of interest in treatment and control groups (or between two treatment groups). Many publications now utilize the terms experimental event rate and control event rate in these calculations:
EER = experimental event rate, i.e. the rate of the event in question in the experimental (treatment) group
CER = control event rate, i.e. the rate of the event in question (death, cure) in the control group
ARR = CER - EER (Note: to change a rate, which is a value among 0 and 1, to a percentage, multiply by 100)
If the the event rate in the treatment group is 10% and event rate in the placebo group is 20%, the ARR is 20% - 10% = 10%. Ensure you keep track of direction, absolute risk reduction or increase are completely different to the patient.
Absolute risk reduction (ARR) is a method of calculating the size of a difference between 2 treatments. It simply informs you how much worse or better one treatment is at decreasing a particular outcome with regards to the actual rates (or numbers) of those who experience the result compared with another treatment.
The ARR is also referred to as the absolute risk difference.
An example of absolute risk reduction
In a clinical trial of a drug to avoid migraines, 2 of 100 individuals taking the drug encounter a migraine (2%), compared with 4 of 100 individuals taking a placebo (4%).
The absolute risk reduction is two%, because 4% - 2% = 2%. That is, there were two% fewer migraines in people using the drug.
It is crucial to know the difference between relative risk reduction and absolute risk reduction. The absolute risk is the simple difference between rates, although relative risk explains the relative or proportional difference.