Understanding Risk, Relative Risk, and Odds Ratios

Risk can be described in a few different ways depending on the type of study. The terms you'll see most often are absolute risk, relative risk, and odds ratio (OR). Here's what each one means.

Absolute Risk

Absolute risk is the chance that something will happen — for example, 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 have a higher relative risk of cancer than non-smokers.

Odds Ratio (OR) in Genetic Studies

In genome-wide association studies (GWAS), researchers usually 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 it.

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. Think of OR = 1 as the baseline (100%). Each increase of 0.1 in OR corresponds to roughly a 10% change in odds, not absolute risk.

Example

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

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