Relative risk (RR) is a statistical measure that compares the likelihood of an outcome in one group versus another. It quantifies how much more probable an event, such as developing a health condition, is in an exposed group compared to an unexposed group. This measure provides a direct comparison of incidence rates between two distinct populations.
Understanding the Equation’s Components
Relative risk calculation relies on data from a 2×2 contingency table. This table categorizes individuals by their exposure status and whether they experienced the outcome. The table includes four values: ‘A’ for exposed individuals who experienced the event, ‘B’ for exposed individuals who did not experience the event, ‘C’ for unexposed individuals who experienced the event, and ‘D’ for unexposed individuals who did not experience the event.
How to Calculate Relative Risk
To calculate relative risk, first determine the incidence of the event in the exposed group. This is done by dividing the number of exposed individuals who experienced the event (A) by the total number of individuals in the exposed group (A + B). For instance, if 30 people out of 100 exposed to a new vitamin developed colds, the incidence in the exposed group would be 30 / 100, or 0.3.
Next, calculate the incidence of the event in the unexposed group. This involves dividing the number of unexposed individuals who experienced the event (C) by the total number of individuals in the unexposed group (C + D). If, in a placebo group of 100 people, 60 developed colds, their incidence would be 60 / 100, or 0.6. Finally, the relative risk is found by dividing the incidence of the exposed group by the incidence of the unexposed group. Using our example, 0.3 divided by 0.6 yields a relative risk of 0.5.
Interpreting the Relative Risk Value
The numerical value obtained from the relative risk calculation provides insights into the relationship between the exposure and the outcome. A relative risk value of 1 indicates that the risk of the outcome is the same for both the exposed and unexposed groups. This suggests that the exposure has no observable effect on the likelihood of the event.
When the relative risk is greater than 1, it signifies an increased risk of the outcome in the exposed group compared to the unexposed group. For example, a relative risk of 3 means that the exposed group is three times more likely to experience the event. Conversely, a relative risk less than 1 suggests a decreased risk in the exposed group, indicating a protective effect. A relative risk of 0.5 implies the exposed group has half the risk of the unexposed group.
Distinguishing Relative and Absolute Risk
Absolute risk refers to the overall probability of an event occurring in a specific population, expressed as a single number or proportion. For instance, the absolute risk of developing a rare disease might be 1 in 1,000,000 individuals over a given period. This value represents the chance of an individual experiencing the event.
A large relative risk does not always translate to a significant absolute risk. Consider an event with an initial absolute risk of 1 in 1,000,000. If an exposure causes the risk to double, the relative risk would be 2. However, the new absolute risk would only increase to 2 in 1,000,000. While the relative increase is substantial, the absolute increase in the number of affected individuals remains small, highlighting the distinct nature of these two risk measures.