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.
