A sample drawn in several phases refers to selectîon of a large-sized preliminary sample ın whiclı some quick and inexpensive metlıod of measurement is applied. A subsample is drawn and a somewhat more elaborate method of measurement is used and then successively smaller sub- samples are drawn and successively more expensive, elaborate and accurate methods of measu­ rement are applied. This paper furnishes estimator and variance expressions for the three-phase design. The examples, arising from remote sensing applications, illustrate estimation of the po- pulation mean of a numerical characteristic and the population of a binary variable of interest. Eormulas for optimum sampling rates are shown to be of the same form for the the two types of variables.