# Variance Estimation in case of nonrespondents

I saw in the book of Rubin (1987) that an increase in variance of estimation will occur due to nonresponses. But I wonder the reason behind this. Thanks for your shares!

The "increase" is relative to a fully responding sample. So if you sample $n$ but only $n_o< n$ units actually respond, then the variance will be higher - just like if you sampled less people.