# Ratio Estimation

INTRODUCTION:

In Survey Sampling, we often use information on some auxiliary variable, to improve our estimator of the finite population parameter, by giving our estimator protection against selection of bad sample. One such estimator is the ratio estimator introduced as follows:

In practice, we are often interested to estimate the ratio of the type:

R= \frac{\bar{Y}}{\bar{X}} = \frac{\sum Y_i}{\sum X_i}

For example, in different socio-economic survey, we may be interested in per-capita expenditure on food-items, infant mortality rate, literacy rate, etc. So, the estimation of R itself is of interest to us and beside that, we can get an improved estimate of Y ̄, as follows:

NOTATIONS:

Since,  \bar{Y} = R \bar{X}  , where R is unknown but  \bar{X}  is known, we can take

\hat{\bar{Y}}_R = \hat{R} \bar{X} = \frac{\bar{y}}{\bar{x}} \bar{X}

as an estimate of  \bar{Y}  and this estimator is called the ratio estimator of  \bar{Y}  .

RESULT 1:

\hat{\bar{Y}_R} is not an unbiased estimator of \hat{\bar{Y}} and its approximate bias is given by:

B(\hat{\bar{Y_R}}) = \frac{1}{\bar{X}} \frac{1-f}{n} \left( RS_X^2 - S_{XY} \right)

;where \quad S_X^2= \frac{1}{N-1} \sum_{i=1}^{N} \left( X_i - \bar{X} \right)^2 , \\
\quad \quad  S_{XY}= \frac{1}{N-1} \sum_{i=1}^{N} \left( X_i - \bar{X} \right) \left(Y_i - \bar{Y} \right)

RESULT 2:

\frac{\left| B( \hat{\bar{Y}}_R) \right|}{ \sigma_{\hat{\bar{Y}}_R}} \leq \left| C.V.(\bar{x}) \right|

RESULT 3:

Mean square error of \hat{\bar{Y}_R} is given by:

MSE(\hat{\bar{Y}}_R) = \frac{1-f}{n} \left[ S_Y^2 + R^2 S_X^2 - 2 R S_{XY}   \right]

RESULT 4:

In SRSWOR, for large n, an approximation of the variance of \hat{\bar{Y}_R} is given by:

V(\hat{\bar{Y}}_R) = \frac{1-f}{n}  \frac{1}{N-1} \sum_{i=1}^{N} U_i^2 \\
\textit{; where} \quad U_i= Y_i-RX_i, \forall i=1(1)N