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2005

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Articles 31 - 60 of 279

Full-Text Articles in Statistics and Probability

Application Of A Variable Importance Measure Method To Hiv-1 Sequence Data, Merrill D. Birkner, Mark J. Van Der Laan Nov 2005

Application Of A Variable Importance Measure Method To Hiv-1 Sequence Data, Merrill D. Birkner, Mark J. Van Der Laan

U.C. Berkeley Division of Biostatistics Working Paper Series

van der Laan (2005) proposed a method to construct variable importance measures and provided the respective statistical inference. This technique involves determining the importance of a variable in predicting an outcome. This method can be applied as an inverse probability of treatment weighted (IPTW) or double robust inverse probability of treatment weighted (DR-IPTW) estimator. A respective significance of the estimator is determined by estimating the influence curve and hence determining the corresponding variance and p-value. This article applies the van der Laan (2005) variable importance measures and corresponding inference to HIV-1 sequence data. In this data application, protease and reverse …

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Casual Mediation Analyses With Structural Mean Models, Thomas R. Tenhave, Marshall Joffe, Kevin Lynch, Greg Brown, Stephen Maisto Nov 2005

Casual Mediation Analyses With Structural Mean Models, Thomas R. Tenhave, Marshall Joffe, Kevin Lynch, Greg Brown, Stephen Maisto

UPenn Biostatistics Working Papers

We represent a linear structural mean model (SMM)approach for analyzing mediation of a randomized baseline intervention's effect on a univariate follow-up outcome. Unlike standard mediation analyses, our approach does not assume that the mediating factor is randomly assigned to individuals (i.e., sequential ignorability). Hence, a comparison of the results of the proposed and standard approaches in with respect to mediation offers a sensitivity analyses of the sequential ignorability assumption. The G-estimation procedure for the proposed SMM represents an extension of the work on direct effects of randomized treatment effects for survival outcomes by Robins and Greenland (1994) (Section 5.0 and …

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Development Of Commercial Applications For Recycled Plastics Using Finite Element Analysis, Nanjunda Narasimhamurthy Nov 2005

Development Of Commercial Applications For Recycled Plastics Using Finite Element Analysis, Nanjunda Narasimhamurthy

Theses and Dissertations

This thesis investigates the suitability of thermo-kinetically recycled plastics for use in commercial product applications using finite element analysis and statistics. Different recycled material blends were tested and evaluated for their use in commercial product applications. There are six different blends of thermo-kinetically recycled plastics used for testing and CATIA is used for finite element analysis. The different types of thermo-kinetically recycled plastics blends are: pop bottles made of PolyethyleneTeraphthalate (PET), milk jugs made of High-Density Polyethylene (HDPE), Vinyl seats made of Poly Vinyl Chloride (PVC) and small amount of Polypropylene (PP) and Urethane, electronic scrap made of engineering resins …

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A General Imputation Methodology For Nonparametric Regression With Censored Data, Dan Rubin, Mark J. Van Der Laan Nov 2005

A General Imputation Methodology For Nonparametric Regression With Censored Data, Dan Rubin, Mark J. Van Der Laan

U.C. Berkeley Division of Biostatistics Working Paper Series

We consider the random design nonparametric regression problem when the response variable is subject to a general mode of missingness or censoring. A traditional approach to such problems is imputation, in which the missing or censored responses are replaced by well-chosen values, and then the resulting covariate/response data are plugged into algorithms designed for the uncensored setting. We present a general methodology for imputation with the property of double robustness, in that the method works well if either a parameter of the full data distribution (covariate and response distribution) or a parameter of the censoring mechanism is well approximated. These …

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Estimating A Treatment Effect With Repeated Measurements Accounting For Varying Effectiveness Duration, Ying Qing Chen, Jingrong Yang, Su-Chun Cheng Nov 2005

Estimating A Treatment Effect With Repeated Measurements Accounting For Varying Effectiveness Duration, Ying Qing Chen, Jingrong Yang, Su-Chun Cheng

UW Biostatistics Working Paper Series

To assess treatment efficacy in clinical trials, certain clinical outcomes are repeatedly measured for same subject over time. They can be regarded as function of time. The difference in their mean functions between the treatment arms usually characterises a treatment effect. Due to the potential existence of subject-specific treatment effectiveness lag and saturation times, erosion of treatment effect in the difference may occur during the observation period of time. Instead of using ad hoc parametric or purely nonparametric time-varying coefficients in statistical modeling, we first propose to model the treatment effectiveness durations, which are the varying time intervals between the …

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Modeling Differentiated Treatment Effects For Multiple Outcomes Data, Hongfei Guo, Karen Bandeen-Roche Nov 2005

Modeling Differentiated Treatment Effects For Multiple Outcomes Data, Hongfei Guo, Karen Bandeen-Roche

Johns Hopkins University, Dept. of Biostatistics Working Papers

Multiple outcomes data are commonly used to characterize treatment effects in medical research, for instance, multiple symptoms to characterize potential remission of a psychiatric disorder. Often either a global, i.e. symptom-invariant, treatment effect is evaluated. Such a treatment effect may over generalize the effect across the outcomes. On the other hand individual treatment effects, varying across all outcomes, are complicated to interpret, and their estimation may lose precision relative to a global summary. An effective compromise to summarize the treatment effect may be through patterns of the treatment effects, i.e. "differentiated effects." In this paper we propose a two-category model …

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Modeling Distributions Of Test Scores With Mixtures Of Beta Distributions, Jingyu Feng Nov 2005

Modeling Distributions Of Test Scores With Mixtures Of Beta Distributions, Jingyu Feng

Theses and Dissertations

Test score distributions are used to make important instructional decisions about students. The test scores usually do not follow a normal distribution. In some cases, the scores appear to follow a bimodal distribution that can be modeled with a mixture of beta distributions. This bimodality may be due different levels of students' ability. The purpose of this study was to develop and apply statistical techniques for fitting beta mixtures and detecting bimodality in test score distributions. Maximum likelihood and Bayesian methods were used to estimate the five parameters of the beta mixture distribution for scores in four quizzes in a …

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Model Evaluation Based On The Distribution Of Estimated Absolute Prediction Error, Lu Tian, Tianxi Cai, Els Goetghebeur, L. J. Wei Nov 2005

Model Evaluation Based On The Distribution Of Estimated Absolute Prediction Error, Lu Tian, Tianxi Cai, Els Goetghebeur, L. J. Wei

Harvard University Biostatistics Working Paper Series

The construction of a reliable, practically useful prediction rule for future response is heavily dependent on the "adequacy" of the fitted regression model. In this article, we consider the absolute prediction error, the expected value of the absolute difference between the future and predicted responses, as the model evaluation criterion. This prediction error is easier to interpret than the average squared error and is equivalent to the mis-classification error for the binary outcome. We show that the distributions of the apparent error and its cross-validation counterparts are approximately normal even under a misspecified fitted model. When the prediction rule is …

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Efficacy Studies Of Malaria Treatments In Africa: Efficient Estimation With Missing Indicators Of Failure, Rhoderick N. Machekano, Grant Dorsey, Alan E. Hubbard Nov 2005

Efficacy Studies Of Malaria Treatments In Africa: Efficient Estimation With Missing Indicators Of Failure, Rhoderick N. Machekano, Grant Dorsey, Alan E. Hubbard

U.C. Berkeley Division of Biostatistics Working Paper Series

Efficacy studies of malaria treatments can be plagued by indeterminate outcomes for some patients. The study motivating this paper defines the outcome of interest (treatment failure) as recrudescence and for some subjects, it is unclear whether a recurrence of malaria is due to that or new infection. This results in a specific kind of missing data. The effect of missing data in causal inference problems is widely recognized. Methods that adjust for possible bias from missing data include a variety of imputation procedures (extreme case analysis, hot-deck, single and multiple imputation), inverse weighting methods, and likelihood based methods (data augmentation, …

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Comparison Of Statistical Tests In Logistic Regression: The Case Of Hypernatreamia, Stylianos Katsaragakis, Christos Koukouvinos, Stella Stylianou, Eleni-Maria Theodoraki, Eleni-Maria Theodoraki Nov 2005

Comparison Of Statistical Tests In Logistic Regression: The Case Of Hypernatreamia, Stylianos Katsaragakis, Christos Koukouvinos, Stella Stylianou, Eleni-Maria Theodoraki, Eleni-Maria Theodoraki

Journal of Modern Applied Statistical Methods

The logistic regression has become an integral component of any medical data analysis concerning binary responses. The main issue rising after the adaptation of the final model is its goodness-of-fit. The fit of the model is assessed via the overall measures and summary statistics and comparing them in the case of hypernateamia.

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An Estimator Of Intervention Effect On Disease Severity, David Siev Nov 2005

An Estimator Of Intervention Effect On Disease Severity, David Siev

Journal of Modern Applied Statistical Methods

When a medical intervention prevents a dichotomous outcome, the size of its effect is often estimated with the prevented fraction. Some interventions may reduce the severity of an outcome without entirely preventing it. To quantify the effect of a severity-moderating intervention, a measure termed the mitigated fraction (MF) is proposed. MF has broad applicability, because it measures the overlap of two empirical distributions based on their stochastic ordering. It is also useful in the specific context of medical interventions, because it shares certain structural and functional features with the prevented fraction. The two measures may be applied together …

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Bootstrap Intervals Of The Parameters Of Lognormal Distribution Using Power Rule Model And Accelerated Life Tests, Mohammed Al-Haj Ebrahem Nov 2005

Bootstrap Intervals Of The Parameters Of Lognormal Distribution Using Power Rule Model And Accelerated Life Tests, Mohammed Al-Haj Ebrahem

Journal of Modern Applied Statistical Methods

Assumed that the distribution of the lifetime of any unit follows a lognormal distribution with parameters μ and σ . Also, assume that the relationship between μ and the stress level V is given by the power rule model. Several types of bootstrap intervals of the parameters were studied and their performance was studied using simulations and compared in term of attainment of the nominal confidence level, symmetry of lower and upper error rates and the expected width. Conclusions and recommendations are given.

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Large Sample And Bootstrap Intervals For The Gamma Scale Parameter Based On Grouped Data, Ayman Baklizi, Amjad Al-Nasser Nov 2005

Large Sample And Bootstrap Intervals For The Gamma Scale Parameter Based On Grouped Data, Ayman Baklizi, Amjad Al-Nasser

Journal of Modern Applied Statistical Methods

Interval estimation of the scale parameter of the gamma distribution using grouped data is considered in this article. Exact intervals do not exist and approximate intervals are needed Recently, Chen and Mi (2001) proposed alternative approximate intervals. In this article, some bootstrap and jackknife type intervals are proposed. The performance of these intervals is investigated and compared. The results show that some of the suggested intervals have a satisfactory statistical performance in situations where the sample size is small with heavy proportion of censoring.

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A Comparison Of The Spearman-Brown And Flanagan-Rulon Formulas For Split Half Reliability Under Various Variance Parameter Conditions, David A. Walker Nov 2005

A Comparison Of The Spearman-Brown And Flanagan-Rulon Formulas For Split Half Reliability Under Various Variance Parameter Conditions, David A. Walker

Journal of Modern Applied Statistical Methods

Differences between the Spearman-Brown and Flanagan-Rulon formulas are examined when the variance parameters for two halves of a test had the following ratios: 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0 and also had a correlation between the two halves of a test at 1.00, .95, .90, .80, .70, .60, .50, .40, .30, .20, .10, .05. It was found that use of the Spearman-Brown formula to estimate the population ρ when the ratio between the standard deviations on two halves of a test is disparate, or beyond .9 to 1.1, was not warranted. Applied and theoretical examples …

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Restricted Quasi-Independent Model Resolves Paradoxical Behaviors Of Cohen’S Kappa, Vicki Stover Hertzberg, Frank Xu, Michael Haber Nov 2005

Restricted Quasi-Independent Model Resolves Paradoxical Behaviors Of Cohen’S Kappa, Vicki Stover Hertzberg, Frank Xu, Michael Haber

Journal of Modern Applied Statistical Methods

Cohen’s kappa, an index of inter-rater agreement, behaves paradoxically in 2×2 tables. λA is derived, an index from the restricted quasi-independent model for 2×2 tables. Simulation studies are used to demonstrate λA has superior performance compared to Scott’s pi. Moreover, λA does not show paradoxical behavior for 2×2 tables.

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Maximum Tests Are Adaptive Permutation Tests, Markus Neuhäeuser, Ludwig A. Hothorn Nov 2005

Maximum Tests Are Adaptive Permutation Tests, Markus Neuhäeuser, Ludwig A. Hothorn

Journal of Modern Applied Statistical Methods

In some areas, e.g., statistical genetics, it is common to apply a maximum test, where the maximum of several competing test statistics is used as a new statistic, and the permutation distribution of the maximum is used for inference. Here, it is shown that maximum tests are special cases of adaptive permutation tests. The 30-year old idea of adaptive statistical tests is more flexible than previously thought when permutation tests are used, and the selector statistic is calculated for every permutation. Because the independence between the selector and the test statistics is no longer needed, the test statistics themselves can …

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Inferences About The Components Of A Generalized Additive Model, Rand R. Wilcox Nov 2005

Inferences About The Components Of A Generalized Additive Model, Rand R. Wilcox

Journal of Modern Applied Statistical Methods

A method for making inferences about the components of a generalized additive model is described. It is found that a variation of the method, based on means, performs well in simulations. Unlike many other inferential methods, switching from a mean to a 20% trimmed mean was found to offer little or no advantage in terms of both power and controlling the probability of a Type I error.

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The Individuals Control Chart In Case Of Non-Normality, BetüL Kan, Berna Yazici Nov 2005

The Individuals Control Chart In Case Of Non-Normality, BetüL Kan, Berna Yazici

Journal of Modern Applied Statistical Methods

This article examines the effects of non-normality as measured by skewness and provides an alternative method of designing individuals control chart with non-normal distributions. A skewness correction method for constructing the individuals control chart is provided. An example of thickness of biscuit process is presented to illustrate the individuals control chart limits.

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An Alternative To Warner’S Randomized Response Model, Sat Gupta, Javid Shabbir Nov 2005

An Alternative To Warner’S Randomized Response Model, Sat Gupta, Javid Shabbir

Journal of Modern Applied Statistical Methods

A modification to Warner’s (1965) Randomized Response Model is suggested. The suggested model is more efficient than the original model.

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Inference For P(Y, Vee Ming Ng Nov 2005

Inference For P(Y, Vee Ming Ng

Journal of Modern Applied Statistical Methods

Some tests and confidence bounds for the reliability parameter R=P(Y

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Determining Parallel Analysis Criteria, Marley W. Watkins Nov 2005

Determining Parallel Analysis Criteria, Marley W. Watkins

Journal of Modern Applied Statistical Methods

Determining the number of factors to extract is a critical decision in exploratory factor analysis. Simulation studies have found the Parallel Analysis criterion to be accurate, but it is computationally intensive. Two freeware programs that implement Parallel Analysis on Macintosh and Windows operating systems are presented.

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Change Point Estimation Of Bilevel Functions, Leming Qu, Yi-Cheng Tu Nov 2005

Change Point Estimation Of Bilevel Functions, Leming Qu, Yi-Cheng Tu

Journal of Modern Applied Statistical Methods

Reconstruction of a bilevel function such as a bar code signal in a partially blind deconvolution problem is an important task in industrial processes. Existing methods are based on either the local approach or the regularization approach with a total variation penalty. This article reformulated the problem explicitly in terms of change points of the 0-1 step function. The bilevel function is then reconstructed by solving the nonlinear least squares problem subject to linear inequality constraints, with starting values provided by the local extremas of the derivative of the convolved signal from discrete noisy data. Simulation results show a considerable …

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Correlation Between The Number Of Epileptic And Healthy Children In Family Size That Follows A Size-Biased Modified Power Series Distribution, Ramalingam Shanmugam, Anwar Hassan, Peer Bilal Ahmad Nov 2005

Correlation Between The Number Of Epileptic And Healthy Children In Family Size That Follows A Size-Biased Modified Power Series Distribution, Ramalingam Shanmugam, Anwar Hassan, Peer Bilal Ahmad

Journal of Modern Applied Statistical Methods

An expression for the correlation between the random number of epileptic and healthy children in family whose size follows a size-biased Modified Power Series Distribution (SBMPSD) is obtained and illustrated. As special cases, results are extracted for size biased Modified Negative Binomial Distribution (SBGNBD), size biased Modified Poisson Distribution (SBGPD) and size biased Modified Logarithmic Series Distribution (SBGLSD).

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Simulation Of Non-Normal Autocorrelated Variables, H.E.T. Holgersson Nov 2005

Simulation Of Non-Normal Autocorrelated Variables, H.E.T. Holgersson

Journal of Modern Applied Statistical Methods

All statistical methods rely on assumptions to some extent. Two assumptions frequently met in statistical analyses are those of normal distribution and independence. When examining robustness properties of such assumptions by Monte Carlo simulations it is therefore crucial that the possible effects of autocorrelation and non-normality are not confounded so that their separate effects may be investigated. This article presents a number of non-normal variables with non-confounded autocorrelation, thus allowing the analyst to specify autocorrelation or shape properties while keeping the other effect fixed.

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Interval Estimation Of Risk Difference In Simple Compliance Randomized Trials, Kung-Jong Lui Nov 2005

Interval Estimation Of Risk Difference In Simple Compliance Randomized Trials, Kung-Jong Lui

Journal of Modern Applied Statistical Methods

Consider the simple compliance randomized trial, in which patients randomly assigned to the experimental treatment may switch to receive the standard treatment, while patients randomly assigned to the standard treatment are all assumed to receive their assigned treatment. Six asymptotic interval estimators for the risk difference in probabilities of response among patients who would accept the experimental treatment were developed. Monte Carlo methods were employed to evaluate and compare the finite-sample performance of these estimators. An example studying the effect of vitamin A supplementation on reducing mortality in preschool children was included to illustrate their practical use.

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A Robust Exponentially Weighted Moving Average Control Chart For The Process Mean, Michael B. C. Khoo, S. Y. Sim Nov 2005

A Robust Exponentially Weighted Moving Average Control Chart For The Process Mean, Michael B. C. Khoo, S. Y. Sim

Journal of Modern Applied Statistical Methods

To date, numerous extensions of the exponentially weighted moving average, EWMA charts have been made. A new robust EWMA chart for the process mean is proposed. It enables easier detection of outliers and increase sensitivity to other forms of out-of-control situation when outliers are present.

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A Comparison Of Risk Classification Methods For Claim Severity Data, Noriszura Ismail, Abdul Aziz Jemain Nov 2005

A Comparison Of Risk Classification Methods For Claim Severity Data, Noriszura Ismail, Abdul Aziz Jemain

Journal of Modern Applied Statistical Methods

The objective of this article is to compare several risk classification methods for claim severity data by using weighted equation which is written as a weighted difference between the observed and fitted values. The weighted equation will be applied to estimate claim severities which is equivalent to the total claim costs divided by the number of claims.

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Supporting And Preparing Future Decision-Makers With The Needed Tools, Michael Wolf-Branigin Nov 2005

Supporting And Preparing Future Decision-Makers With The Needed Tools, Michael Wolf-Branigin

Journal of Modern Applied Statistical Methods

Supporting and Preparing Future Decision-makers with the Needed ToolsEducational and social service researchers and evaluators continue to develop advanced statistical methods. To ensure that our students have the essential skills as they enter direct service, the focus must be on assuring that they learn readily understandable methods that are appropriate for small samples and use repeated measures.

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Statistical Tests, Tests Of Significance, And Tests Of A Hypothesis Using Excel, David A. Heiser Nov 2005

Statistical Tests, Tests Of Significance, And Tests Of A Hypothesis Using Excel, David A. Heiser

Journal of Modern Applied Statistical Methods

Microsoft’s spreadsheet program Excel has many statistical functions and routines. Over the years there have been criticisms about the inaccuracies of these functions and routines (see McCullough 1998, 1999). This article reviews some of these statistical methods used to test for differences between two samples. In practice, the analysis is done by a software program and often with the actual method used unknown. The user has to select the method and variations to be used, without full knowledge of just what calculations are used. Usually there is no convenient trace back to textbook explanations. This article describes the Excel algorithm …

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Obituary: Cliff Lunneborg, Jmasm Editors Nov 2005

Obituary: Cliff Lunneborg, Jmasm Editors

Journal of Modern Applied Statistical Methods

No abstract provided.

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