A procedure that is akin to that employed to estimate a population proportion p is ratio estimation. It can be used to determine a population ratio proper or to estimate a population mean. Ratio estimation involves taking measurements on two characteristics of interest of a sampling unit—one corresponding to the numerator and the other corresponding to the denominator of the ratio. In this regard, let a sample of size n consist of the n ordered pairs of observations (X_{i}, Y_{i},), i = 1,. . .,n, where the sample values Y_{1},. . .,Y_{n} refer to some measurable characteristic of the sampling unit that appears in the numerator of the ratio, and the sample values X_{1},. . .,X_{n} pertain to some measurable characteristic of the sampling unit that appears in the denominator of the ratio. In what follows, the X_{i}'s will serve as subsidiary information used for ratio estimation purposes.

At this point, some notation is in order. Let the population ratio appear as follows:

where the population totals for the Y- and X-characteristics are denoted as and , respectively, and and are the respective means for the said characteristics. Then an estimator for R is the sample ...

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