ABSTRACT
Besides y there may be related variables x, z, . . ., called auxiliary variables, with values
X1, X2, . . . , X N ; Z1, Z2, . . . , ZN ; . . . respectively, for the units of U . These values may be partly or fully known to the investigator; if the values of an auxiliary variable are positive, this variable may be called a size measure of the units of U .
