Efficacy; effectiveness; attributable effect

 Effect size is a measure of how effective a treatment is when applied to a group of patients. It can be used to measure by how much a group of patients improves after treatment, or by how much a particular treatment is better than another treatment to which it is compared.

For any outcome variable, a group of patients will typically exhibit a normal distribution of values (Fig. 1). That distribution will have a mean value (µ) and a standard deviation (sd). About 68% of the patients will express a value between one standard deviation less than the mean value, and one standard deviation greater. The outcome measure may be pain scores or any other outcome of interest.

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