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Absolute risk difference

The absolute risk difference is the absolute additional risk of an event due to an exposure. It is the risk in the exposed group minus the risk in the unexposed (or differently exposed) group. If the exposure is harmful (e.g. effect of smoking on risk of developing lung cancer) then the absolute risk difference is sometimes called the absolute risk excess and it represents the absolute increase in risk for those exposed compared to the unexposed. If, however, the exposure is considered to be protective (eg effect of taking folic acid supplements during pregnancy to reduce the risk of neural tube defects) then the absolute risk difference is known as the absolute risk reduction.

Absolute Risk Difference = Pexposed - Punexposed

Below are the results of a clinical trial examining whether patients with chronic fatigue syndrome (CFS) improved six weeks after treatment with intramuscular magnesium. The group who received the magnesium were compared to a group who received a placebo and the outcome was feeling better

 

Number who had magnesium

Number who had placebo

Total

Number who felt better

12

3

15

Number who did not feel better

3

14

17

Total

15

17

32

Risk 'improvement' on magnesium = 12 / 15 = 0.80

Risk of 'improvement' on placebo = 3 / 17 = 0.18

Absolute risk reduction = 0.80 - 0.18 = 0.62