**Unplanned domain structure**

A domain is defined
unplanned, if the *domain sample size n _{sd} is not fixed in the sampling design*.
This is the case in which the desired domain structure is not a part of the sampling
design. Thus, the domain sample sizes are random quantities introducing an increase
in the variance estimates of domain estimators. In addition, an extremely small
number (even zero) of sample elements in a domain can be realized in this
case, if the domain size in the population is small.
See pages 188-190 in Lehtonen, R & Pahkinen, E. (2004)