Glossary term
Relative Risk
Relative risk compares the probability of an outcome in one group with the probability of the same outcome in another group.
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What Is Relative Risk?
Relative risk compares the probability of an outcome in one group with the probability of the same outcome in another group. It tells whether one group is more or less likely to experience the outcome than the comparison group.
Relative risk is useful for comparison, but it can be misleading when shown without absolute risk. A risk that doubles may still be small if the baseline risk was tiny.
Key Takeaways
- Relative risk compares risk between two groups.
- A relative risk above 1 means the event is more likely in the first group.
- A relative risk below 1 means the event is less likely in the first group.
- Relative risk does not show the actual probability by itself.
- Absolute risk is needed to understand practical importance.
How Relative Risk Works
Relative risk is calculated by dividing the risk in one group by the risk in another group. If one group has a 10% event rate and another has a 5% event rate, the relative risk is 2. The first group is twice as likely to experience the event.
That does not mean the event is likely in an everyday sense. It means the first group has twice the risk of the comparison group over the defined period and under the defined conditions.
How to Read the Number
Relative risk | Interpretation |
|---|---|
1.0 | No difference between the groups. |
Above 1.0 | Higher risk in the first group. |
Below 1.0 | Lower risk in the first group. |
2.0 | Twice the risk of the comparison group. |
0.5 | Half the risk of the comparison group. |
Example
If an insurance claim occurs in 4% of one group and 2% of another, the relative risk is 2. The first group has twice the risk. The absolute difference, however, is 2 percentage points. A decision-maker should know both.
In investing, a strategy may have twice the drawdown risk of another strategy but still have a low absolute probability of a severe loss in normal conditions. In health or insurance decisions, a relative increase may sound alarming until the baseline is shown.
Where It Can Mislead
Relative risk can exaggerate emotional impact when the starting risk is low. A risk rising from 1 in 10,000 to 2 in 10,000 has doubled, but the absolute increase is still 1 in 10,000. The same relative risk can be much more important when the baseline risk is high.
It can also hide severity. Two events may have the same relative risk but very different consequences. A small increase in the risk of a catastrophic loss may matter more than a large increase in the risk of a minor inconvenience.
Relative risk is persuasive because it is simple. “Twice as likely” is easier to remember than a full probability table. That simplicity is useful, but it can also be used in marketing, policy debate, and investment commentary to make a small effect sound large.
Good risk communication pairs relative risk with the starting probability. If one strategy reduces the chance of a loss from 10% to 5%, the relative reduction is 50% and the absolute reduction is 5 percentage points. If it reduces risk from 0.2% to 0.1%, the relative reduction is still 50%, but the practical effect is much smaller.
Relative risk can also vary by subgroup. A risk comparison that is meaningful for one age, income, health, credit, or portfolio group may not apply to another. The comparison group matters as much as the ratio.
The Bottom Line
Relative risk is a comparison tool. It helps show how one risk compares with another, but it should be read with absolute risk, time frame, baseline conditions, and the financial consequence of the outcome.