Some populations are difficult to sample because their members constitute a small part of a larger population and are not readily identifiable. Examples include millionaires, persons with diabetes, or crime victims. This chapter describes methods that can be used when the population of interest is rare or hard to find.

In other situations, estimates are desired for subpopulations such as individual states, but the survey's sample sizes for some states are too small to construct reliable estimates. Small area estimation techniques rely on models to obtain estimators for areas where the sample size is too small to estimate the quantity of interest directly.