Risk Difference

Using the 2×2 table (Please see Aschengrau & Seage, Chapter 3):

Disease + Disease - Total
Exposure + a b a+b
Exposure - c d c+d
Total a+c b+d a+b+c+d

Probability of Disease in the Exposed: a/a+b
Probability of Disease in the Unexposed: c/c+d

The risk difference is the absolute difference between two risks, usually exposed - unexposed:

RD = [a/a+b] - [c/c+d]

The risk difference measures clinical and public health importance of the causal relationship.