2-12 Relative measures of effects can be misleading

Relative measures of effects (e.g. the ratio of the probability of an outcome in one treatment group compared with that in a comparison group) are insufficient for judging the importance of the difference (between the probabilities 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 a treatment reduces the probability of getting an illness by 50% but also has harms, and your risk of getting the illness is 2 in 100, receiving the treatment is likely to be worthwhile. If, however, your risk of getting the illness is 2 in 10,000, then receiving the treatment is unlikely to be worthwhile even though the relative effect is the same.

Always consider the absolute effects of treatments – that is, the difference in outcomes between the treatment groups being compared. Do not make a treatment decision based on relative effects alone.

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Know Your Chances

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