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  • 301.
    Öhman, Marie-Louise
    Umeå University, Faculty of Social Sciences, Department of Statistics.
    Contributions to generalized Wilcoxon rank tests1992Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    In unbalanced small sample size problems with right censorings, the variance estmators of linear rank statistics, may be biased. This is the case with the commonly applied variance estimators for the generalized Wilcoxon rank test statistics of Gehan and Prentice. The bias and variance of different variance estimators and observed significance level and power for standardized tests are compared in a Monte Carlo simulation study. The variance estimators are the permutational-, the conditional permutational- and the jackknife variance estimators for the statistic Gehan, and the asymptotic- and the jackknife variance estimators for the statistic of Prentice. It appears that observed level and power and variance properties may be improved by using the jackknife variance estimator.

    Further, The sensitivity curves of Turkey are used to establish the sensitivity to gross errors and misclassifications for standardized generalized Wilcoxon rank sum statistics in small samples with right censorings. For a certain combined sample, which might contain gross errors, a relatively fast method is needed to establish the applicability of the inference drawn from the selected ranktest. Ome way is to use the "change of decision point" (cdp), which in this thesis is defined as the smallest proportion of altered positions resulting in an opposite decision.

  • 302.
    Öhman, Marie-Louise
    Umeå University, Faculty of Social Sciences, Department of Statistics.
    Estimating location parameters using censored rank statistics1995In: Biometrical Journal, ISSN 0323-3847, E-ISSN 1521-4036, Vol. 37, no 7, p. 767-782Article in journal (Refereed)
    Abstract [en]

    In this paper censored data rank location estimators are obtained by using censored one-sample rank test statistics of the location parameter and censored two-sample rank test statistics of the shift of location parameter. Also, methods for constructing censored small sample confidence intervals and asymptotic confidence intervals for the location are considered. Generalizations of the solutions from uncensored one-sample and two-sample rank tests are utilized.

  • 303.
    Öhman, Marie-Louise
    Umeå University, Faculty of Social Sciences, Department of Statistics.
    Generalized Wilcoxon rank tests and sensitivity curves1991In: Biometrical Journal, ISSN 0323-3847, E-ISSN 1521-4036, Vol. 33, no 7, p. 789-806Article in journal (Refereed)
    Abstract [en]

    In this paper we use the sensitivity curves of TUKEY (1977) and the change of decision point (cdp) (a modified version of the breakdown point of YLVISAKER, 1977), supplemented by simulation studies to acquire knowledge about sensitivity in generalized Wilcoxon rank test statistics. Sensitivity depends on balanced or unbalanced sample size cases, censoring, combinations of failure distributions and sources of errors in the data. It is important to consider the quality of the data, and the results show that cdp and some properties of the sensitivity curves may serve as a hint when selecting a test statistic and when making a decision for a given test statistic.

4567 301 - 303 of 303
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