Design effect

In statistics, the design effect (or estimates of unit variance) is an adjustment used in some kinds of studies, such as cluster randomised trials, to allow for the design structure. The adjustment inflates the variance of parameter estimates, and therefore their standard errors, which is necessary to allow for correlations among clusters of observations. It is similar to the variance inflation factor and is used in sample size calculations. The term was introduced by Leslie Kish in 1965.