RLO: Relative Risk Reduction (RRR) and Absolute Risk Reduction (ARR)

Relative Risk and Absolute Risk - Trial One

Consider the following situation.

Two randomised, controlled trials for a new drug, Curit, are conducted in two different population groups of patients at a risk of a heart attack, over a period of 10 years.

The first trial is conducted in a group of 100 patients who have several risk factors for a heart attack and are classed as having a high baseline risk.

Without treatment, 90 patients out of 100 will experience a heart attack over the next 10 years. The control event rate (CER) is 90%.

With Curit, the number of high risk patients who will experience a heart attack over the next 10 years falls to 60 patients out of 100. The experimental event rate (EER) is 60%

We can present the magnitude of effect with Curit in one of two ways:

Relative risk reduction for trial 1
The relative risk reduction (RRR), represents the difference in event rates expressed relative to (or proportionally of) the control event rate. It is usually expressed as a percentage.
RRR = (CER - EER) out of CER.
“Curit reduces the event rate of a heart attack over the next 10 years by one-third or by 33%”
Absolute risk reduction for trial 1
The absolute risk reduction (ARR), represents the difference in event rates between the experimental group and the control group. It is also usually expressed as a percentage.
ARR = CER - EER
“Curit will save 30% of patients from a heart attack over the next 10 years that would have otherwise occurred”

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