ABSTRACT
If ancillary information is available for each of the units of the universe, it can, under suitable conditions, be used in several ways to improve the efficiency of the estimators of the study variable. One way, to be described in this chapter, is to use the ancillary information, after sample selection, in providing what are known as ratio estimators and regression estimators. The other, described in Chapter 5, is to use the ancillary information in sample selection and also in estimation. The two could be used in combination for two ancillary variables.
