Relative risk (RR):  Also called the risk ratio. The RR can be calculated only after you’ve done a prospective or experimental study. Relative risk or risk ratio (RR) is the ratio of the probability of an event occurring (for example, developing a disease, being injured) in an exposed group to the probability of the event occurring in a comparison, non-exposed group.

RR = (Disease risk in the exposed group) / ( disease risk in unexposed group). In other words,
RR = (Probability of disease happening in exposed group) ÷ (Probability of disease happening in non-exposed group)

For example,  e.g., 45% risk of pancreatic cancer in exposed vs. 15% risk in unexposed gives a 3 times increase in RR in the exposed group. A relative risk of 1 suggests no association between exposure & outcome. A relative risk of > 1 or < 1 is clinically significant. A RR of less than one means the event is less likely to occur in the intervention group. A relative risk of greater than one means the event is more likely in the intervention group.

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