Clinicians may be more inclined to prescribe an intervention that reduces the risk of death by 25% than one that reduces the risk of death by 1 percentage point, although both presentations of the evidence may relate to the same benefit (i.e. a reduction in risk from 4% to 3%). The former refers to the relative reduction in risk and the latter to the absolute reduction in risk. As described in Chapter 9 (Section 9.2.2), there are several measures for comparing dichotomous outcomes in two groups. Meta-analyses are usually undertaken using risk ratios (RR), odds ratios (OR) or risk differences (RD), but there are several alternative ways of expressing results.
Relative risk reduction (RRR) is a convenient way of re-expressing a risk ratio as a percentage reduction:
RRR = 100% × (1 – RR).
For example, a risk ratio of 0.75 translates to a relative risk reduction of 25%, as in the example above.
The risk difference is often referred to as the absolute risk reduction (ARR), and may be presented as a percentage (for example, 1%), as a decimal (for example, 0.01), or as counts (for example, 10 out of 1000). A simple transformation of the risk difference known as the number needed to treat (NNT) is a common alternative way of presenting the same information. We discuss NNTs in Section 12.5.2, and consider different choices for presenting absolute effects in Section 12.5.3. We then describe computations for obtaining these numbers from the results of individual studies and of meta-analyses.