The relative efficiency (RE) of using the finite m imputation estimator, rather than using an infinite number for the fully efficient imputation, in units of variance, is approximately a function of m and (Rubin 1987, p. 114):
where mis the number of imputations and is the fraction of missing information.
Table 75.7 shows relative efficiencies with different values of m and .
Table 75.7: Relative Efficiencies



m 
10% 
20% 
30% 
50% 
70% 

3 
0.9677 
0.9375 
0.9091 
0.8571 
0.8108 

5 
0.9804 
0.9615 
0.9434 
0.9091 
0.8772 

10 
0.9901 
0.9804 
0.9709 
0.9524 
0.9346 

20 
0.9950 
0.9901 
0.9852 
0.9756 
0.9662 
The table shows that if the fraction of missing information is modest, only a small number of imputations are needed. For example, if , only three imputations are needed to have a 91% efficiency and five imputations are needed to have a 94% efficiency.