Absolute risk reduction (ARR)

Absolute risk reduction (ARR) is a way of measuring the size of a difference between two treatments. It simply tells you how much better or worse one treatment is at reducing a particular outcome in terms of the actual numbers (or rates) of people who experience the outcome compared with another treatment.

The ARR is also called the ‘absolute risk difference’.

An example of absolute risk reduction

In a clinical trial of a drug to prevent migraines, 2 of 100 people taking the drug experience a migraine (2%), compared with 4 of 100 people taking a placebo (4%).

The absolute risk reduction is 2%, because 4% − 2% = 2%. That is, there were 2% fewer migraines in people taking the drug.

It is important to understand the difference between absolute risk reduction and relative risk reduction (RRR). The absolute risk is the simple difference between rates, while relative risk describes the relative or proportional difference.

An example of relative risk reduction

Using the above example, 4% of people taking placebo had a migraine, but only 2% of those taking the drug. In other words, the risk of a migraine was 50% lower for people taking the drug.

Notice that a 50% difference sounds more impressive than a 2% difference, but both these numbers describe the same difference in effect, just in a different way. Whether it is a meaningful difference or not depends on other factors, such as the consequences of the event, the cost of the medicine and any unwanted effects.

Knowing the real difference makes a difference. Using a non-scientific example, suppose the price of a handbag is reduced by 50%. That sounds like a big saving. However, suppose the original price was $20,000 and it is now $10,000. In real terms, the cost is still more than most people would pay for a handbag.