Understanding Risk, Relative Risk, and Odds Ratios

Scientists and clinicians describe risk in a few different ways, depending on the type of study being used. The most common terms you’ll see are absolute risk, relative risk, and odds ratio (OR).

Absolute Risk

Absolute risk is the chance that something will happen.

For example, it’s the probability that a person will develop a condition such as diabetes over a certain period of time.

Relative Risk

Relative risk compares the chance of something happening between two groups.

For example, if smokers develop cancer more often than non-smokers, smokers are said to have a higher relative risk of cancer compared to non-smokers.

Odds Ratio (OR) in Genetic Studies

In genome-wide association studies (GWAS), researchers typically use odds ratios (OR) instead of relative risk.

An odds ratio compares:

• the odds of having a trait if you carry a specific genetic variant (allele)

  versus

• the odds of having that trait if you do not carry that allele

How to Interpret Odds Ratios

• OR = 1: The allele has no effect on the trait (baseline risk).
• OR < 1: The allele is associated with lower odds of the trait.
• OR > 1: The allele is associated with higher odds of the trait.

The farther the odds ratio is from 1, the stronger the association. You can think of OR = 1 as the baseline (100%).

Each increase of 0.1 in OR corresponds to approximately a 10% change in odds, not absolute risk.

Example

If a minor allele has an OR of 1.2, this means:

• The odds of having the trait are 20% higher for someone with that allele compared to someone without it.
• In other words, a person with the minor allele is 1.2 times as likely to have the trait compared to someone with two copies of the major allele.

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