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Experimental event rate

Wikipedia contributorsAdapted for Fathomly · View authors
1 min read
Source updated September 17, 2025

In epidemiology and biostatistics, the experimental event rate (EER) is a measure of how often a particular statistical event (such as response to a drug, adverse event or death) occurs within the experimental group (non-control group) of an experiment.

This value is very useful in determining the therapeutic benefit or risk to patients in experimental groups, in comparison to patients in placebo or traditionally treated control groups.

Three statistical terms rely on EER for their calculation: absolute risk reduction, relative risk reduction and number needed to treat.

01Control event rate

The control event rate (CER) is identical to the experimental event rate except that is measured within the scientific control group of an experiment.

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This article is adapted from the Wikipedia article “Experimental event rate”, written by its contributors and licensed under CC BY-SA 4.0. Fathomly has changed the layout, removed citation markers, navigation and maintenance notices, and adjusted punctuation. This adapted version is shared under the same license. For references, see the original article.

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Related topics

Risk difference

The risk difference, excess risk, or attributable risk is the difference between the risk of an outcome in the exposed group and the unexposed group. It is computed as I e − I u {\displaystyle I_{e}-I_{u}} , where I e {\displaystyle I_{e}} is the incidence in the exposed group, and I u {\displaystyle I_{u}} is the incidence in the unexposed group.

Relative risk reduction

In epidemiology, the relative risk reduction or efficacy is the relative decrease in the risk of an adverse event in the exposed group compared to an unexposed group. It is computed as ( I u − I e ) / I u {\displaystyle (I_{u}-I_{e})/I_{u}} , where I e {\displaystyle I_{e}} is the incidence in the exposed group, and I u {\displaystyle I_{u}} is the incidence in the unexposed group.

Number needed to treat

The number needed to treat or number needed to treat for an additional beneficial outcome (NNTB) is an epidemiological measure used in communicating the effectiveness of a health-care intervention, typically a treatment with medication. The NNT is the average number of patients who need to be treated to prevent one additional bad outcome.