On the Effect of Stratification When Two Stratifying Variables Are Used.

Most of the literature on survey sampling deals with a single stratifying variable. In this paper an attempt is made to study the effect of using two stratifying variables. We present an approximation to the variance of the study variable under the assumption of a linear regression on the two stratifying variables. This approximation depends only on the number of strata, the simultaneous density of the stratifying variables, and the correlations between the study variable and each of the stratifying variables. The results indicate that in many practical situations the gain from using two stratifying variables over one is nontrivial. [ABSTRACT FROM AUTHOR]