= Design effect
{title2=$D=1+(m-1)\rho$}
{wiki}
The <design effect> is a sampling <variance> under a complex design divided by the corresponding simple-random-sampling <variance>. For independent equal-size clusters of size $m$ with exchangeable <intraclass correlation> $\rho$, a common mean-<estimation> approximation is $1+(m-1)\rho$. It expresses the loss of effective <sample size> from positive correlation; finite-population and unequal-cluster-size designs require their own calculation.
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