Misleading description

Relative effects

Look out for study results that are described as relative effects. Relative effects of interventions alone can be misleading.

Relative effects (e.g. the ratio of the probability of an outcome in one intervention group compared with that in a comparison group) are insufficient for judging the importance of the difference (between the frequencies of the outcome). A relative effect may give the impression that a difference is larger than it actually is when the likelihood of the outcome is small to begin with. For example, if an intervention reduces the probability of an animal getting a disease by 50% but also has harms, and the risk of getting the disease is 2 in 100, receiving the intervention is likely to be worthwhile. If, however, the risk of getting the disease is 2 in 10,000, then receiving the intervention is unlikely to be worthwhile even though the relative effect is the same. The absolute effect of an intervention is likely to vary for subjects with different baseline risks.

REMEMBER: Always consider the absolute effects of interventions – that is, the difference in outcomes between the intervention groups being compared. Avoid making an intervention decision based on relative effects alone.