On a new estimator for the variance of the ratio estimator with small sample corrections
Section 4. Conclusions

In this paper we have derived a new approximation formula for MSE ( Y ¯ ^ R ) MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiFu0Je9sqqrpepC0xbbL8F4rqqrpgpC0xc9LqFf0xc9 qqpeuf0xe9q8qiYRWFGCk9vi=dbbf9v8Gq0db9qqpm0dXdHqpq0=vr 0=vr0=edbaqaaeGaciGaaiaabeqaamaabaabaaGcbaaeaaaaaaaaa8 qacaqGnbGaae4uaiaabweacaaMc8Uaaeika8aadaqiaaqaa8qaceWG zbWdayaaraaacaGLcmaadaWgaaWcbaWdbiaadkfaa8aabeaak8qaca qGPaaaaa@3E89@ of order 1 / n 2 MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiFu0Je9sqqrpepC0xbbL8F4rqqrpgpC0xc9LqFf0xc9 qqpeuf0xe9q8qiYRWFGCk9vi=dbbf9v8Gq0db9qqpm0dXdHqpq0=vr 0=vr0=edbaqaaeGaciGaaiaabeqaamaabaabaaGcbaaeaaaaaaaaa8 qadaWcgaqaaiaaigdaaeaacaWGUbWdamaaCaaaleqabaWdbiaaikda aaaaaaaa@38E3@ and a new formula for the bias of s e ^ 2 MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiFu0Je9sqqrpepC0xbbL8F4rqqrpgpC0xc9LqFf0xc9 qqpeuf0xe9q8qiYRWFGCk9vi=dbbf9v8Gq0db9qqpm0dXdHqpq0=vr 0=vr0=edbaqaaeGaciGaaiaabeqaamaabaabaaGcbaaeaaaaaaaaa8 qacaWGZbWdamaaDaaaleaapeGabmyza8aagaqcaaqaa8qacaaIYaaa aaaa@3930@ of order 1 / n . MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiFu0Je9sqqrpepC0xbbL8F4rqqrpgpC0xc9LqFf0xc9 qqpeuf0xe9q8qiYRWFGCk9vi=dbbf9v8Gq0db9qqpm0dXdHqpq0=vr 0=vr0=edbaqaaeGaciGaaiaabeqaamaabaabaaGcbaaeaaaaaaaaa8 qadaWcgaqaaiaaigdaaeaacaWGUbaaaiaac6caaaa@388D@ The new estimator MSE ^ 2 ( Y ¯ ^ R ) MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiFu0Je9sqqrpepC0xbbL8F4rqqrpgpC0xc9LqFf0xc9 qqpeuf0xe9q8qiYRWFGCk9vi=dbbf9v8Gq0db9qqpm0dXdHqpq0=vr 0=vr0=edbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaecaaeaaqa aaaaaaaaWdbiaab2eacaqGtbGaaeyraaWdaiaawkWaamaaBaaaleaa peGaaGOmaaWdaeqaaOWdbiaaykW7caqGOaWdamaaHaaabaWdbiqadM fapaGbaebaaiaawkWaamaaBaaaleaapeGaamOuaaWdaeqaaOWdbiaa bMcaaaa@407B@ which takes into account the bias of s e ^ 2 MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiFu0Je9sqqrpepC0xbbL8F4rqqrpgpC0xc9LqFf0xc9 qqpeuf0xe9q8qiYRWFGCk9vi=dbbf9v8Gq0db9qqpm0dXdHqpq0=vr 0=vr0=edbaqaaeGaciGaaiaabeqaamaabaabaaGcbaaeaaaaaaaaa8 qacaWGZbWdamaaDaaaleaapeGabmyza8aagaqcaaqaa8qacaaIYaaa aaaa@3930@ appears to be less biased than MSE ^ 0 ( Y ¯ ^ R ) = Var ^ ( Y ¯ ^ R ) MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiFu0Je9sqqrpepC0xbbL8F4rqqrpgpC0xc9LqFf0xc9 qqpeuf0xe9q8qiYRWFGCk9vi=dbbf9v8Gq0db9qqpm0dXdHqpq0=vr 0=vr0=edbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaecaaeaaqa aaaaaaaaWdbiaab2eacaqGtbGaaeyraaWdaiaawkWaamaaBaaaleaa peGaaGimaaWdaeqaaOWdbiaaykW7caqGOaWdamaaHaaabaWdbiqadM fapaGbaebaaiaawkWaamaaBaaaleaapeGaamOuaaWdaeqaaOWdbiaa bMcacqGH9aqppaWaaecaaeaapeGaaeOvaiaabggacaqGYbaapaGaay PadaWdbiaaykW7caqGOaWdamaaHaaabaWdbiqadMfapaGbaebaaiaa wkWaamaaBaaaleaapeGaamOuaaWdaeqaaOWdbiaabMcaaaa@4B35@ and MSE ^ 1 ( Y ¯ ^ R ) . MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiFu0Je9sqqrpepC0xbbL8F4rqqrpgpC0xc9LqFf0xc9 qqpeuf0xe9q8qiYRWFGCk9vi=dbbf9v8Gq0db9qqpm0dXdHqpq0=vr 0=vr0=edbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaecaaeaaqa aaaaaaaaWdbiaab2eacaqGtbGaaeyraaWdaiaawkWaamaaBaaaleaa peGaaGymaaWdaeqaaOWdbiaaykW7caqGOaWdamaaHaaabaWdbiqadM fapaGbaebaaiaawkWaamaaBaaaleaapeGaamOuaaWdaeqaaOWdbiaa bMcacaGGUaaaaa@412C@ For n 8 , MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiFu0Je9sqqrpepC0xbbL8F4rqqrpgpC0xc9LqFf0xc9 qqpeuf0xe9q8qiYRWFGCk9vi=dbbf9v8Gq0db9qqpm0dXdHqpq0=vr 0=vr0=edbaqaaeGaciGaaiaabeqaamaabaabaaGcbaaeaaaaaaaaa8 qacaWGUbGaeyyzImRaaGioaiaacYcaaaa@3A42@ the bias of MSE ^ 2 ( Y ¯ ^ R ) MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiFu0Je9sqqrpepC0xbbL8F4rqqrpgpC0xc9LqFf0xc9 qqpeuf0xe9q8qiYRWFGCk9vi=dbbf9v8Gq0db9qqpm0dXdHqpq0=vr 0=vr0=edbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaecaaeaaqa aaaaaaaaWdbiaab2eacaqGtbGaaeyraaWdaiaawkWaamaaBaaaleaa peGaaGOmaaWdaeqaaOWdbiaaykW7caqGOaWdamaaHaaabaWdbiqadM fapaGbaebaaiaawkWaamaaBaaaleaapeGaamOuaaWdaeqaaOWdbiaa bMcaaaa@407B@ was in all cases of the simulation study less than 7% which is much better than the standard variance estimator; in most cases, this result even holds for n 4. MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiFu0Je9sqqrpepC0xbbL8F4rqqrpgpC0xc9LqFf0xc9 qqpeuf0xe9q8qiYRWFGCk9vi=dbbf9v8Gq0db9qqpm0dXdHqpq0=vr 0=vr0=edbaqaaeGaciGaaiaabeqaamaabaabaaGcbaaeaaaaaaaaa8 qacaWGUbGaeyyzImRaaGinaiaac6caaaa@3A40@ For very small n , MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiFu0Je9sqqrpepC0xbbL8F4rqqrpgpC0xc9LqFf0xc9 qqpeuf0xe9q8qiYRWFGCk9vi=dbbf9v8Gq0db9qqpm0dXdHqpq0=vr 0=vr0=edbaqaaeGaciGaaiaabeqaamaabaabaaGcbaaeaaaaaaaaa8 qacaWGUbGaaiilaaaa@37BA@ MSE ^ 2 ( Y ¯ ^ R ) MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiFu0Je9sqqrpepC0xbbL8F4rqqrpgpC0xc9LqFf0xc9 qqpeuf0xe9q8qiYRWFGCk9vi=dbbf9v8Gq0db9qqpm0dXdHqpq0=vr 0=vr0=edbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaecaaeaaqa aaaaaaaaWdbiaab2eacaqGtbGaaeyraaWdaiaawkWaamaaBaaaleaa peGaaGOmaaWdaeqaaOWdbiaaykW7caqGOaWdamaaHaaabaWdbiqadM fapaGbaebaaiaawkWaamaaBaaaleaapeGaamOuaaWdaeqaaOWdbiaa bMcaaaa@407B@ may have a large negative bias if the population has a large coefficient of variation C x . MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiFu0Je9sqqrpepC0xbbL8F4rqqrpgpC0xc9LqFf0xc9 qqpeuf0xe9q8qiYRWFGCk9vi=dbbf9v8Gq0db9qqpm0dXdHqpq0=vr 0=vr0=edbaqaaeGaciGaaiaabeqaamaabaabaaGcbaaeaaaaaaaaa8 qacaWGdbWdamaaBaaaleaapeGaamiEaaWdaeqaaOGaaiOlaaaa@38F2@ From our simulation study this issue appears to be unlikely to occur as long as C x < 0 .8 . MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiFu0Je9sqqrpepC0xbbL8F4rqqrpgpC0xc9LqFf0xc9 qqpeuf0xe9q8qiYRWFGCk9vi=dbbf9v8Gq0db9qqpm0dXdHqpq0=vr 0=vr0=edbaqaaeGaciGaaiaabeqaamaabaabaaGcbaaeaaaaaaaaa8 qacaWGdbWdamaaBaaaleaapeGaamiEaaWdaeqaaOWdbiabgYda8iaa bcdacaqGUaGaaeioaiaac6caaaa@3C25@

Finally, recall that for the populations in this simulation study, the bias of the ratio estimator itself was consistently small, even for n = 4. MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiFu0Je9sqqrpepC0xbbL8F4rqqrpgpC0xc9LqFf0xc9 qqpeuf0xe9q8qiYRWFGCk9vi=dbbf9v8Gq0db9qqpm0dXdHqpq0=vr 0=vr0=edbaqaaeGaciGaaiaabeqaamaabaabaaGcbaaeaaaaaaaaa8 qacaWGUbGaeyypa0JaaGinaiaac6caaaa@3980@ In general, for other populations this bias may not be negligible. Cochran (1977, pages 174-175) discusses several alternative ratio estimators that are unbiased.

References

Cochran, W.G. (1977). Sampling Techniques. New York: John Wiley & Sons, Inc.

David, I.P., and Sukhatme, B.V. (1974). On the bias and mean square error of the ratio estimator. Journal of the American Statistical Association, 69, 464-466.

Kendall, M.G., and Stuart, A. (1958). The Advanced Theory of Statistics, Volume I. London: Charles Griffin and Company.

Kish, L. (1995). Survey Sampling. New York: John Wiley & Sons, Inc.

Koop, J.C. (1968). An exercise in ratio estimation. The American Statistician, 22, 29-30.

Nath, S.N. (1968). On product moments from a finite universe. Journal of the American Statistical Association, 63, 535-541.

Rao, J.N.K. (1969). Ratio and regression estimators. In New Developments in Survey Sampling, (Eds., N.L. Johnson and H. Smith), New York: John Wiley & Sons, Inc., 213-234.

Sukhatme, P.V. (1954). Sampling Theory of Surveys with Applications, Iowa State College Press, Ames, IA.

Tin, M. (1965). Comparison of some ratio estimators. Journal of the American Statistical Association, 60, 294-307.


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