Risk difference

Risk difference (RD), also known as absolute risk reduction (ARR), is a measure used in epidemiological studies to show the difference in risk between two groups. It is calculated by subtracting the risk of an event in the control group from the risk in the treatment group.

Calculation

The formula for calculating risk difference is:

RD = Ie - Ic

where:

• Ie is the incidence in the exposed group
• Ic is the incidence in the non-exposed group

Interpretation

The risk difference can be interpreted as the proportion of risk that is reduced by the intervention or exposure. A positive risk difference indicates that the risk is higher in the exposed group, while a negative risk difference indicates that the risk is lower in the exposed group.

Use in Clinical Trials

In clinical trials, the risk difference is often used to determine the number of patients that need to be treated in order to prevent one additional bad outcome, known as the number needed to treat (NNT). The NNT is the reciprocal of the risk difference.

Limitations

While the risk difference is a useful measure, it has some limitations. It does not take into account the baseline risk of the population, and it can be influenced by confounding factors. Additionally, it does not provide information about the relative magnitude of the effect of the intervention or exposure.

See Also

• Relative risk
• Odds ratio
• Number needed to harm
• Number needed to treat

Risk difference Resources
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