PMC4336134_01
Two hypothetical trials: the risk ratio is the generalizable measure of the effect of treatment. In both trials, the biological effect of the treatment is the same, corresponding to a one third reduction in the risk of death. The relative risk is the same in both trials and reflects the biological effect of the treatment. However, the risk difference is not the same. The relative risk is a generalizable measure of the biological effect of a treatment, whereas the risk difference is not generalizable.
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PMC4336134_01_40560_2014_56_Fig1_HTML.jpg
Fig1
Figure 1 shows two hypothetical clinical trials.
Hypothetical trial: effect on all-cause mortality is a weighted average of effect on cause-specific mortality. The treatment reduces the risk of death due to bleeding by one third (RR = 0.67) but has no effect on other causes of death (RR = 1.0). The effect on all-cause mortality (RR = 0.8) is a weighted average of its effect on deaths due to bleeding and on other causes of death, weighted according to the relative contributions of the different causes.
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PMC4336134_01_40560_2014_56_Fig2_HTML.jpg
Fig2
Figure 2 shows results from a hypothetical clinical trial of treatment for bleeding.