# How to Calculate Risk Difference

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Risk difference is an important tool that can help you decide whether an experimental intervention has a favorable effect on the risk of a process or treatment. All you will need to determine this figure is reliable data regarding your experiment. Once you obtain the risk difference, you will be able to see what effect the proposed change would have and if the change is worth applying.

### Things You'll Need

• Control group
• Variable group

Form a control group and variable group to determine the risk difference of the experimental intervention. The risk difference will be determined by subtracting the risk of the control from the risk of the variable.

Calculate the risk for each group. This can be achieved by using a 2 by 2 box, which contains the following information for each group:

1) Event (still infected) 2) No event (still not infected) 3) Sum of the above two outcomes

Calculate the following ratios:

(Number of Event in variable/Number of Sum in variable) = risk in variable (Number of Event in control/Number of Sum in control) = risk in control

Use the figures obtained in the previous step to calculate the risk difference. Use the following equation:

Risk difference = risk in variable – risk in control

An outcome that is less than 0 indicates the experimental intervention lowers risk; an outcome greater than 0 means the intervention increases risk.

## Tips & Warnings

• The risk difference cannot be less than -1 or greater than 1.
• An outcome of 0 means the experimental intervention does not change risk.

## References

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