Design effect


In statistics, the design effect is an adjustment used in some kinds of studies, such as ones that use cluster sampling or cluster randomised controlled trial, 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.

Definition

The design effect is the ratio of two theoretical variances for an estimator:
For data collected using cluster sampling with m observations in each cluster and of, the design effect. Deff, is given by: