[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Epidemiology](https://durgical.com/en/epidemiology/) › Bias, confounding and causation # Bias, confounding and causation Selection and information bias, confounding, effect modification and the criteria for causation. 17 pages ## Measure - [**Additive and multiplicative interaction** Two factors together can add or multiply their effects](https://durgical.com/en/epidemiology/additive-and-multiplicative-interaction/) ## Bias - [**Attrition bias** If the people who drop out of a study differ between groups, the result among those who stay can…](https://durgical.com/en/epidemiology/attrition-bias/) - [**Confounding** A third factor, linked to both exposure and outcome, creates or hides an association that is not…](https://durgical.com/en/epidemiology/confounding/) - [**Information (measurement) bias** If exposure or disease is measured badly, people land in the wrong box and the association is…](https://durgical.com/en/epidemiology/information-bias/) - [**Observer bias and the Hawthorne effect** Knowing who is in each group changes how things are measured; feeling watched changes how people…](https://durgical.com/en/epidemiology/observer-bias-and-the-hawthorne-effect/) - [**Recall bias** People who are ill tend to search for and remember their past exposures more than healthy people…](https://durgical.com/en/epidemiology/recall-bias/) - [**Selection bias** It happens when the people who get into or stay in a study do not represent the population of…](https://durgical.com/en/epidemiology/selection-bias/) - [**Spectrum bias** A diagnostic test looks better than it is when it is tried only on very sick and clearly healthy…](https://durgical.com/en/epidemiology/spectrum-bias/) - [**Verification (work-up) bias** If only people who test positive get the reference test, sensitivity looks higher and…](https://durgical.com/en/epidemiology/verification-bias/) ## Concept - [**Bradford Hill criteria for causation** Nine viewpoints, proposed in 1965, for judging whether an association is causal](https://durgical.com/en/epidemiology/bradford-hill-criteria-for-causation/) - [**Causal models: necessary, sufficient and component causes** Almost no disease has a single cause: several pieces act together](https://durgical.com/en/epidemiology/causal-models-necessary-sufficient-and-component-causes/) - [**Chance, bias, confounding or cause** When two things show up together in a study there are four possible explanations](https://durgical.com/en/epidemiology/chance-bias-confounding-or-cause/) - [**Controlling confounding** Confounding can be prevented when designing a study (chance, restriction, matching) or corrected…](https://durgical.com/en/epidemiology/controlling-confounding/) - [**Effect modification (interaction)** The exposure has a different effect depending on a third factor (age, sex, genes)](https://durgical.com/en/epidemiology/effect-modification/) - [**Gene-environment interaction** An exposure harms people with a certain gene variant far more, or a gene only causes disease…](https://durgical.com/en/epidemiology/gene-environment-interaction/) - [**Internal and external validity** Internal: whether the result is true for those who took part](https://durgical.com/en/epidemiology/internal-and-external-validity/) - [**Reverse causation** Sometimes the exposure does not cause the disease; the disease (or its first symptoms) changes…](https://durgical.com/en/epidemiology/reverse-causation/) ## A to Z - [A](https://durgical.com/en/epidemiology/a-z/a/) - [B](https://durgical.com/en/epidemiology/a-z/b/) - [C](https://durgical.com/en/epidemiology/a-z/c/) - [D](https://durgical.com/en/epidemiology/a-z/d/) - [E](https://durgical.com/en/epidemiology/a-z/e/) - [F](https://durgical.com/en/epidemiology/a-z/f/) - [G](https://durgical.com/en/epidemiology/a-z/g/) - [H](https://durgical.com/en/epidemiology/a-z/h/) - [I](https://durgical.com/en/epidemiology/a-z/i/) - [K](https://durgical.com/en/epidemiology/a-z/k/) - [L](https://durgical.com/en/epidemiology/a-z/l/) - [M](https://durgical.com/en/epidemiology/a-z/m/) - [N](https://durgical.com/en/epidemiology/a-z/n/) - [O](https://durgical.com/en/epidemiology/a-z/o/) - [P](https://durgical.com/en/epidemiology/a-z/p/) - [Q](https://durgical.com/en/epidemiology/a-z/q/) - [R](https://durgical.com/en/epidemiology/a-z/r/) - [S](https://durgical.com/en/epidemiology/a-z/s/) - [T](https://durgical.com/en/epidemiology/a-z/t/) - [U](https://durgical.com/en/epidemiology/a-z/u/) - [V](https://durgical.com/en/epidemiology/a-z/v/) - [W](https://durgical.com/en/epidemiology/a-z/w/) - [Y](https://durgical.com/en/epidemiology/a-z/y/) [← Epidemiology, biostatistics and public health](https://durgical.com/en/epidemiology/) Educational content on epidemiological and statistical methods; the examples use made-up numbers to teach the calculation. Not a clinical or legal guide: current Colombian rules are in the official sources. Representative model, for educational purposes only. --- Source: https://durgical.com/en/epidemiology/areas/bias/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/epidemiology/areas/bias/