| Outcome | |||
| Positive Effect | Negative Effect | Risk | |
| Treated (y) | a | b | Y=a/a+b |
| Control (x) | c | d | X=c/c+d |
The absolute risk reduction is the difference in risk between the control group (x) and the treated group (y):
The relative risk, or risk ratio, is the ratio of risk in the treated group (y) to risk in the control group (x):
The relative risk reduction expresses the percent reduction in events in treated (y) compared to controls (x):
The number needed to treat is the inverse of the absolute risk reduction: