  In many surveys inexpensive auxiliary variables are available that can help
us to make more precise estimation about the main variable. Using auxiliary
variable has been extended by regression estimators for rare and cluster
populations. In conventional regression estimator it is assumed that the mean
of auxiliary variable in the population is known. In many surveys we don't have
such wide information about auxiliary variable. In this paper we present a
multi-phase variant of two-stage sequential sampling based on an inexpensive
auxiliary variable associated with the survey variable in the form of double
sampling. The auxiliary variable will be used in both design and estimation
stage. The population mean is estimated by a modified regression-type estimator
with two different coefficient. Results will be investigated using some
simulations following Median and Thompson (2004).
