Relative Risk & Odds Ratio

Scientists and doctors measure risk as either relative risk or absolute risk.

Absolute risk is the chance of something happening—for example, the probability of getting diabetes.

Relative risk is the chance of something happening in one group compared to another. For instance, smokers are more likely to develop cancer compared to non-smokers. Smokers are therefore said to have a higher relative risk of cancer.

In GWAS, researchers calculate odds ratios (OR) rather than relative risk. The OR measures the chance of finding the trait if you have the allele compared to if you don't have the allele.

• If an OR is equal to 1, then the allele has no effect on the trait.
• If an OR is lower than 1, the effect allele decreases the odds of the trait.
• If an OR is higher than 1, the effect allele increases the odds of the trait.

The strength of the effect depends on how far the odds ratio is from OR = 1. You can think of OR = 1 as 100% or the baseline.

Every increase of 0.1 represents a 10% increase in risk.

For example, a minor allele with OR = 1.2 increases the odds by 20% compared to the major allele. Someone with the minor allele would be 1.2 times as likely to have the trait compared to someone with two copies of the major allele.