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  • 1. Brännäs, E.
    et al.
    Brännäs, Kurt
    Umeå University, Faculty of Social Sciences, Umeå School of Business and Economics (USBE), Economics.
    A model of patch visit behaviour in fish1998In: Biometrical Journal, ISSN 0323-3847, E-ISSN 1521-4036, Vol. 40, no 6, p. 717-724Article in journal (Refereed)
    Abstract [en]

    The patch visit or sampling behaviour of fish is studied over a 15 day period. The experimental setup consists of three connected patches between which movement is automatically registered. Using an integer-valued time series model we find a diel effect on the probability to stay in one of the patches.

  • 2.
    Brännäs, Kurt
    et al.
    Umeå University, Faculty of Social Sciences, Umeå School of Business and Economics (USBE), Statistics.
    Pohjanpalo, J
    Small sample properties of the likelihood estimator in proportional hazard regression-models with omitted variables1986In: Biometrical Journal, ISSN 0323-3847, E-ISSN 1521-4036, Vol. 28, no 2, p. 219-228Article in journal (Refereed)
  • 3.
    Doretti, Marco
    et al.
    Department of Statistics, Computer Science, and Applications, University of Florence, Florence, Italy.
    Genbäck, Minna
    Umeå University, Faculty of Social Sciences, Umeå School of Business and Economics (USBE), Statistics.
    Stanghellini, Elena
    Umeå University, Faculty of Social Sciences, Umeå School of Business and Economics (USBE). Department of Economics, University of Perugia, Perugia, Italy.
    Mediation analysis with case–control sampling: identification and estimation in the presence of a binary mediator2024In: Biometrical Journal, ISSN 0323-3847, E-ISSN 1521-4036, Vol. 66, no 1, article id 2300089Article in journal (Refereed)
    Abstract [en]

    With reference to a stratified case–control (CC) procedure based on a binary variable of primary interest, we derive the expression of the distortion induced by the sampling design on the parameters of the logistic model of a secondary variable. This is particularly relevant when performing mediation analysis (possibly in a causal framework) with stratified case–control (SCC) data in settings where both the outcome and the mediator are binary. Despite being designed for parametric identification, our strategy is general and can be used also in a nonparametric context. With reference to parametric estimation, we derive the maximum likelihood (ML) estimator and the M-estimator of the joint outcome–mediator parameter vector. We then conduct a simulation study focusing on the main causal mediation quantities (i.e., natural effects) and comparing M- and ML estimation to existing methods, based on weighting. As an illustrative example, we reanalyze a German CC data set in order to investigate whether the effect of reduced immunocompetency on listeriosis onset is mediated by the intake of gastric acid suppressors.

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  • 4.
    Öhman, Marie-Louise
    Umeå University, Faculty of Social Sciences, Department of Statistics.
    A Monte Carlo study of some censored data Wilcoxon rank tests1990In: Biometrical Journal, ISSN 0323-3847, E-ISSN 1521-4036, Vol. 32, no 6, p. 721-735Article in journal (Refereed)
    Abstract [en]

    The variance estimators usually applied for the generalized censored data Wilcoxon rank tests by Gehan and Peto & Prentice, are heavily biased in unbalanced problems. This paper reports the results of a Monte Carlo simulation study, where jackknifing is used to construct estimators of variance. Size, power and variance properties are compared for five variance estimators, when using different combinations of group sizes, failure and censoring patterns. The variance estimators are the permutational, the conditional permutational and the jackknife variance estimators for the statistic of Gehan and the asymptotic and the jackknife variance estimators for the statistic of Peto & Prentice. It appears that observed size, power and variance properties may be improved by using the jackknife variance estimator, when comparing to the variance estimators usually applied.

  • 5.
    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.

  • 6.
    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.

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