5.5.3. Relative versus absolute reductions

The recommended output of a meta-analysis is a relative risk or odds ratio as they are more stable across different baseline risks, but absolute reductions are more useful for clinical decisions. The valid way of estimating the absolute risk reduction is to multiply the relative risk from the meta-analysis with the most plausible estimate or range of estimates of the baseline risk, this is the risk in the assumed control group, often called the assumed control risk.

Assumed control risk (ACR):

As relative risks are more stable, absolute risk reductions are best calculated applying a relative risk reduction to an assumed control risk. Determining the assumed control risk is not always easy and straightforward. The best estimates usually come from Belgian data, e.g. for cancer the data from the cancer registry are a valuable tool. When not available one can use expert opinion or take the baseline effect from studies in the literature. When different studies exist with varying baseline risk sometimes the median control risk is taken (this is what GRADE pro does by default), but it may be better to report a range of plausible values.