oddssuhde

Oddssuhde, or odds ratio, is a measure of association between an exposure and an outcome used in statistics and epidemiology. It compares the odds that the outcome occurs in the exposed group to the odds in the unexposed group.

In a simple 2x2 table, let a be the number of exposed individuals with the outcome, b the number of exposed without the outcome, c the number of unexposed with the outcome, and d the number of unexposed without the outcome. The odds ratio is calculated as OR = (a × d) / (b × c). The odds of the outcome in the exposed group are a/b, and in the unexposed group are c/d; the odds ratio is their ratio.

Interpretation follows that OR > 1 indicates higher odds of the outcome with exposure, OR < 1 indicates lower odds, and OR = 1 suggests no association. When the outcome is rare, the odds ratio approximates the relative risk, but they are not the same and should not be interpreted interchangeably.

Estimation and uncertainty are commonly expressed with a confidence interval, often derived from the natural logarithm of the odds ratio. A 95% interval is typically reported as exp[ln(OR) ± 1.96 × SE], where SE is the standard error of ln(OR). Logistic regression is a frequent means of estimating ORs, especially when adjusting for multiple covariates.

Commonly used in case-control studies and in genetics or epidemiology, the odds ratio is a versatile but sometimes non-intuitive measure. It should be interpreted in the context of study design and alongside other measures such as relative risk or risk differences.