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relative risk ( RR )

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Relative risk is the ratio between the rate of disease in the population exposed to a particular factor and the rate in those not exposed.

  • Given the following:
    • a = suspected cause present and disease present
    • b = suspected cause present and disease absent
    • c = suspected cause absent and disease present
    • d = suspected cause absent and disease absent
    • total disease present = a+c
    • total disease absent = b+d
    • total suspected cause present = a+b
    • total suspected cause absent = c+d

  • relative risk (RR) = a / (a+b) divided by the result of c/ (c+d)

If:

the RR =1, or the confidence interval (CI) includes 1, then there is no significant difference between the treatment and control groups

the RR>1 and the CI does not include 1, events are significantly more likely in the treatment group than the control group

the RR<1 and CI does not include 1, events are significantly less likely in the treatment group than the control group

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