**Ratio Estimation of Population Total**

Ratio estimation can be used to improve the efficiency of the estimation
of T, if a continuous auxiliary variable z is available. The population total T_{z} and
the n sample values z_{k} of z are required for this method. This
information can be used to improve the estimation of T by first calculating
the sample estimator of the ratio R = T/T_{z} and multiplying it by the
known total T_{z}. Ratio estimation of the total can be very efficient if the ratio
Y_{k}/Z_{k} of the values of the study and auxiliary variables is nearly constant across
the population.
Ratio estimators are usually effective but slightly biased.
See pages 93-95 in Lehtonen, R & Pahkinen, E. (2004)