Effective size = ( ∑ i = 1 n w i ) 2 ∑ i = 1 n w i 2 {\displaystyle ={\frac {(\sum _{i=1}^{n}w_{i})^{2}}{\sum _{i=1}^{n}w_{i}^{2}}}}
where w i {\displaystyle w_{i}} is the weight of the ith respondent.
