**Unit Nonresponse**

All intended measurements for a selected sample element could be missing, e.g. owing to a
refusal to participate in a personal interview.
Unit nonresponse results in a sample data set whose size n_{(r)} is smaller than the
intended sample size n, thus increasing the standard errors of the estimates.
See pages 113-114 in Lehtonen, R & Pahkinen, E. (2004)