Measures of frequency and association
Incidence, prevalence, relative risk, odds ratio, risk reduction, NNT and NNH.
13 pages
Measure
- Absolute risk reductionThe difference in risk between the control and treated groups: how many events are prevented per…
- Attributable risk and attributable fractionHow much of the risk in the exposed is due to the exposure, as cases per 1,000 and as a…
- IncidenceHow many new cases appear in a group that was disease-free at the start, over a defined time
- Incidence rate and person-timeNew cases divided by the total time each person was observed; it is used when follow-up differs…
- Number needed to harmHow many people must be treated, over a given time, for one to suffer an adverse effect caused…
- Number needed to treatHow many people must be treated, over a given time, to prevent one more event than without…
- Odds ratioCompares the odds of having been exposed in cases and in controls; it is the measure of…
- Population attributable riskHow many cases in the whole population are due to an exposure; it depends on its strength and on…
- PrevalenceThe share of a population that has the disease at a point or over a period, counting new and…
- Relative riskHow many times more (or less) often the exposed become ill compared with the unexposed
- Relative risk reductionWhat share of the original risk a treatment removes; it always sounds bigger than the absolute…
Concept
- Prevalence, incidence and durationHow many people are ill depends on how many fall ill and how long the illness lasts: P ≈ I × D
- The 2×2 tableA four-cell table that crosses exposure (or a test result) with disease; almost every measure in…
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← Epidemiology, biostatistics and public health
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.