[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Epidemiology](https://durgical.com/en/epidemiology/) › Measures of frequency and association # Measures of frequency and association Incidence, prevalence, relative risk, odds ratio, risk reduction, NNT and NNH. 13 pages ## Measure - [**Absolute risk reduction** The difference in risk between the control and treated groups: how many events are prevented per…](https://durgical.com/en/epidemiology/absolute-risk-reduction/) - [**Attributable risk and attributable fraction** How much of the risk in the exposed is due to the exposure, as cases per 1,000 and as a…](https://durgical.com/en/epidemiology/attributable-risk-and-attributable-fraction/) - [**Incidence** How many new cases appear in a group that was disease-free at the start, over a defined time](https://durgical.com/en/epidemiology/incidence/) - [**Incidence rate and person-time** New cases divided by the total time each person was observed; it is used when follow-up differs…](https://durgical.com/en/epidemiology/incidence-rate-and-person-time/) - [**Number needed to harm** How many people must be treated, over a given time, for one to suffer an adverse effect caused…](https://durgical.com/en/epidemiology/number-needed-to-harm/) - [**Number needed to treat** How many people must be treated, over a given time, to prevent one more event than without…](https://durgical.com/en/epidemiology/number-needed-to-treat/) - [**Odds ratio** Compares the odds of having been exposed in cases and in controls; it is the measure of…](https://durgical.com/en/epidemiology/odds-ratio/) - [**Population attributable risk** How many cases in the whole population are due to an exposure; it depends on its strength and on…](https://durgical.com/en/epidemiology/population-attributable-risk/) - [**Prevalence** The share of a population that has the disease at a point or over a period, counting new and…](https://durgical.com/en/epidemiology/prevalence/) - [**Relative risk** How many times more (or less) often the exposed become ill compared with the unexposed](https://durgical.com/en/epidemiology/relative-risk/) - [**Relative risk reduction** What share of the original risk a treatment removes; it always sounds bigger than the absolute…](https://durgical.com/en/epidemiology/relative-risk-reduction/) ## Concept - [**Prevalence, incidence and duration** How many people are ill depends on how many fall ill and how long the illness lasts: P ≈ I × D](https://durgical.com/en/epidemiology/prevalence-incidence-and-duration/) - [**The 2×2 table** A four-cell table that crosses exposure (or a test result) with disease; almost every measure in…](https://durgical.com/en/epidemiology/the-2-2-table/) ## 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/measures/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/epidemiology/areas/measures/