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

## Measures of Effect

We can illustrate how measures of effect can be expressed in different ways if we consider buying a lottery ticket. Let's assume that chance of buying a winning ticket is 1 in 20 million. What happens if we buy two lottery tickets?

Consider the following statements:

• "If I buy two lottery tickets I will double my chances of winning, increasing my chances by 100%."
• "If I buy two lottery tickets, my chance of winning is 2 in 20 million, so an absolute increase in my chances of winning is 0.0001%"

Although the statements represent the same information, it has simply been expressed in different ways. As we can see, the figures appear much more impressive in the first statement. Similarly, information within heatlhcare can be presented in different ways.

Benefits of a therapy can be expressed in terms of the relative risk reduction (RRR) and the absolute risk reduction (ARR).

Harms of a therapy can be also be expressed in terms of the relative risk increase (RRI) and the absolute risk increase (ARI).

This learning object will focus on the methods of measuring the benefits of a therapy (RRR and ARR) but the principles apply to both benefits and harms (adverse effects).

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