bluuebunny/crossref_metadata_2025_split huggingface dataset:
Advances in statistical methodology for diagnostic medecine ni the 1980's
Authors: Begg Colini B. 
Published: April 2007

AbstractResearch on diagnostic medicine has been directed at a number of topics in the past decade. Issues which have received a lot of attention are ROC analysis and the identification and correction of various analytic biases. Other topics of widespread interest include the use of expert systems, the relationship of such systems to statistical data‐based systems, and the evaluation of tests using cost‐effectiveness analysis. Increasingly there is a sentiment that well‐designed, prospective trials are required to provide credible information on the accuracy of diagnostic technologies, and so a consensus on methodological standards is needed, paralleling the earlier development of such standards in clinical trials and epidemiology.
Assessment of quality of life in clinical trials
Authors: Schumacher Martin , Olschewski Manfred , Schulgen Gabi 
Published: April 2007

AbstractThis paper is intended as an overview of developments in the assessment of quality of life (QOL) in clinical trials over the last decade from the viewpoint of clinical biostatistics. In the first part we deal with aspects of obtaining adequate measurements of quality of life. A literature survey shows that a large number of quite heterogeneous measurement approaches for use in clinical trials exist, a substantial percentage of which cannot be regarded as sufficient for their actual measuring purpose. In the second part we review statistical methods applied to and adapted for the analysis of QOL data. Underlying the analysis should be the assumption of QOL as a stochastic process. Applied analysis procedures are again investigated in a literature survey. Finally, critical conclusions are outlined and suggestions for further research are given.
Comparing in‐patient classification systems: A problem of non‐nested regression models
Authors: Willan Andrew R. , Ross William , Mackenzie Thomas A. 
Published: April 2007

AbstractSince 1983, hospitals in the United States have been receiving prospective payment for their in‐hospital patient admissions covered under Medicare. Under such schemes each patient is placed in a group by a classification system, known as the Diagnosis Related Groups (DRG), and the hospital is reimbursed by the Health Care Financing Administration according to some predetermined group average, adjusted for hospital level characteristics, such as size, location and teaching activity. Recent interest has focused on refining the DRG system or considering totally different systems of classification. Studies designed to compare the ability of different systems to account for between‐patient variability in resource consumption in the same dataset lead to the problem of model selection between large non‐nested regressions, where resource consumption, measured by length of hospital stay or costs, is regressed on dummy‐indicator variables representing different patient groups. We use a simple measure of fit to develop a symmetric test of the null hypothesis that the two systems account equally well for variability in resource consumption. With this method, unlike methods such as Akaike's AIC criterion, we can quantify the probability of a false positive, and thereby limit the probability of choosing one system over another when it is no better at accounting for variability in resource consumption.
Withdrawal censoring and the sequential logrank procedure
Authors: Richardson De Juran 
Published: April 2007

AbstractThe effects of withdrawal censoring on the power of the group sequential logrank procedure are examined in the context of survival studies. Special attention is given to an assumption that the withdrawal censoring mechanism is only conditionally independent of survival time. Simulation studies of two‐sample, fixed duration trials, under random right censoring, staggered entry and exponential life times, reveal a general decrease in statistical power that is most pronounced when the withdrawals are unbalanced, the majority occurring in the group with the higher hazard rate. Generally, this decrease in power is not of practical concern under typical withdrawal rates except when conditional independence is present in which case large fluctuations in power and type I error rates can occur.
Short and medium term projections of the aids/hiv epidemic by a dynamic model with an application to the risk group of homo/bisexual men in amsterdam
Authors: Heisterkamp S. H. , de Haan B. J. , Jager J. C. , van Druten J. A. M. , Hendriks J. C. M. 
Published: April 2007

AbstractSeveral methods exist for short term projection of the numbers of AIDS cases. Some use extrapolation of empirical curves fitted to data up to a given time, whereas others such as the popular method of ‘back projection’ or actuarial methods also use information about the process. In this paper we describe a dynamic model based on a distributed modelling technique allowing for variability both in infectiousness and in age distribution of the population at risk. Some model parameters are taken from the literature, others are estimated from AIDS incidence data from the homo/bisexual population in Amsterdam. The model described here simulates prevalence and incidence of HIV infection. We present prediction intervals for two years from January 1990 onwards. We discuss three scenarios based on the estimated model, two of which consider early treatment with anti‐viral drugs. Given the model and the state of the epidemic in Amsterdam, early treatment intervention must be combined with very drastic measures for reducing infectivity in order to have any serious impact on the course of the epidemic.
Analysis of disease risks using ancillary risk factors, with application to job–exposure matrices
Authors: Gilks Walter R. , Richardson Sylvia 
Published: April 2007

AbstractEpidemiological studies of disease can make use of ancillary risk‐factors, acquired from individuals outside the disease study. For example, several disease studies might use the same job‐exposure matrix to quantify risks due to occupational exposure to industrial agents. We construct a graphical model to combine a logistic regression disease model with models for the ancillary data and the risk‐factor distribution in the population. We estimate the graphical model using Gibbs sampling, and in simulations compare it with methods of direct substitution into logistic regression.
Methods for comparing cumulative hazard functions in a semi‐proportional hazard model
Authors: Dabrowska Dorota M. , Doksum Kjell A. , Feduska Nicholas J. , Husing Robert , Neville Padraic 
Published: April 2007

AbstractGraphical methods based on the analysis of differences between log cumulative hazard functions are considered for a two‐group semi‐proportional hazard model which allows for interaction between treatments and covariates. Confidence procedures and test statistics that can be used to test for interaction and for main effects are developed. Their use is illustrated by applying them to the analysis of kidney transplant data from the University of California, San Francisco.
Evaluations of methods for statistical surveillance
Authors: Frisén M. 
Published: April 2007

AbstractThe needs for statistical surveillance in different areas of medicine are described. The predictive value of an alarm and other measures for the evaluation of alarm procedures are suggested. The measures are used to evaluate some common methods of continual surveillance of time series. It is demonstrated that some methods have about the same properties at the start of the surveillance period as later. This is however not the case for all methods. For some methods the consequence of an early alarm should be quite different from that of a late one.
On the sample size for studies based upon mcnemar's test
Authors: Lachenbruch Peter A. 
Published: April 2007

AbstractWhen computing the sample size for studies using McNemar's test, one needs to know the probability of discordance and the odds ratio to be detected. In many studies, the investigator is unable to specify the probability of discordance, but can state, at least approximately, the marginal probabilities of each variable. This information leads to restrictions on the possible values of the cell probabilities and provides a range of admissible values for the off‐diagonal cells. We compute the sample size needed in these circumstances and compare them to the results cited by Schlesselman and Connett et al. These sample sizes for the method are quite close to those found in the Monte Carlo study of Connett et al.
Using mark‐recapture methodology to estimate the size of a population at risk for sexually transmitted diseases
Authors: Rubin Gail , Umbach David , Shyu Shwu‐Fang , Castillo‐Chavez Carlos 
Published: April 2007

AbstractTo study the spread of sexually transmitted diseases (STDs) using social/sexual mixing models, one must have quantitative information about sexual mixing. An unavoidable complication in gathering such information by survey is that members of the surveyed population will almost certainly have sexual contacts outside that population. The number of these outsiders may be substantial and, hence, important for the modelling process. In this paper, we develop a mark‐recapture model for estimating the size of the population at risk for contracting a STD due to direct sexual contact with a specified population targeted by a survey. This mark‐recapture methodology provides a reliable method of estimating the number of outsiders. Because not everyone in the targeted population may be sexually active, the size of the sexually active subset, used as the number marked in our tag‐recapture formulation, must be estimated, which introduces extra variability. We derive an estimator of the variance of the estimated total number at risk that accounts for this extra variability and an expression for the bias of that estimator. We extend the methodology to stratified surveys and illustrate its use with data collected from a population of university undergraduates to estimate sexual mixing parameters of a deterministic model of the spread of STDs.
Effects of mid‐point imputation on the analysis of doubly censored data
Authors: Law C. Gordon , Brookmeyer Ron 
Published: April 2007

AbstractDoubly censored data arise in some cohort studies of the AIDS incubation period because the time of infection may be known only up to an interval defined by two successive screening tests for HIV antibody. A simple analytic approach is to impute the infection time by the mid‐point of the interval and then apply standard survival techniques for right censored data. The objective of this paper is to investigate the statistical properties of such a mid‐point imputation approach. We investigated the asymptotic bias of the Kaplan‐Meier estimate, coverage probabilities of associated confidence intervals, bias in hazard ratio, and the size of the logrank test. We show that the statistical properties of mid‐point imputation depend strongly on the underlying distributions of infection times and the incubation periods, and the width of the interval between screening tests. In the absence of treatment, the median incubation period of HIV infection is approximately 10 years, and we conclude that, for this situation, mid‐point imputation is a reasonable procedure for interval widths of 2 years or less.
Estimation of a common risk ratio in stratified case‐cohort studies
Authors: Sato Tosiya 
Published: April 2007

AbstractThe case‐cohort study design is a useful modification of the case‐control design, which allows direct estimation of the risk ratio without the rare‐disease assumption. While several risk ratio estimation procedures have been proposed under large‐strata settings, only the Mantel‐Haenszel point estimator is available in sparse stratifications. This paper provides simple confidence limits methods, based on the large‐sample distribution of the Mantel‐Haenszel risk ratio, that apply to both large‐strata and sparse‐data situations. For the Tarone risk ratio, I give a new large‐strata variance estimate.
Correlated binomial variates: Properties of estimator of intraclass correlation and its effect on sample size calculation
Authors: Feng Ziding , Grizzle James E. 
Published: April 2007

AbstractIn group randomized studies, the sample size calculations are complicated by within group (worksite, community, etc.) correlation. We compare by simulation the moment method and the more standard ANOVA method of estimating the intraclass correlation. We find the former is less biased for a small to moderate number of clusters but the difference disappears when the appropriate degree of freedom is used for the ANOVA estimator. We propose a simulation approach for sample size determination and illustrate it with an example.
A note on last case influence using the partial likelihood approach under heavy censoring
Authors: Oman Paul 
Published: April 2007

AbstractThe Cox proportional hazards model always treats the last observation in a time ordered data set as an inexact survival time. In certain situations this can lead to this observation always having a low case influence and consequently never being highlighted by a case influence analysis. Thus an influential value can become non‐influential by a miscalculation of survival time and this, in turn, can affect conclusions of an analysis based on statistical significance (P‐values).
Estimating hiv prevalence and projecting aids incidence in the united states: A model that accounts for therapy and changes in the surveillance definition of aids
Authors: Rosenberg P. S. , Gail M. H. , Carroll R. J. 
Published: April 2007

AbstractThe AIDS incubation distribution is changing in calendar time because of treatment and changes in the surveillance definition of AIDS. To obtain reliable estimates of HIV prevalence and projections of AIDS incidence in the 1990s using the method of backcalculation, we constructed an appropriate incubation distribution for each calendar date of infection.We parameterized the impact of treatment on the incubation distribution by specifying the relative hazard for AIDS in treated versus untreated people as a function of duration of HIV infection. To account for trends in the incubation distribution, we modelled the prevalence of treatment, the distribution of treatment onset times, and the impact of the revision of the AIDS surveillance definition in 1987. We selected and evaluated backcalculation models based on consistency with external information. We defined a ‘plausible range’ of estimates that took into account uncertainty about the natural incubation distribution and treatment efficacy, as well as bootsrap assessment of stochastic error.Using these methods, we projected that national United States AIDS incidence will plateau during 1991–1994 at over 50,000 caes per year. Projections exhibited substantial systematic uncertainty, and we calculated a plausible range for AIDS incidence in 1994 of 42,300 to 70,700 cases. An estimated 628,000 to 988,000 cumulative HIV infections occurred as of 1 January 1991. After accounting for AIDS mortality, we estimated that 484,000 to 844,000 people were living the HIV infection on 1 January 1991. Favourable trends in HIV incidence appeared in gay men and intravenous drug users. Plausible ranges for our estimates overlapped with those from a ‘stage model’ approach to incorporating treatment effects in backcalculations. Our approach, however, tended to yield smaller estimates of epidemic size, mainly because the parameters used with the stage model implied that more treatment was in use and that treatment was more effective than in our model.
Impact of measurement error and temporal variability on the estimation of event probabilities for risk factor intervention trials
Authors: Neaton James D. , Bartsch Glenn E. 
Published: April 2007

AbstractThe impact of measurement error and temporal variability of risk factors on estimates of disease probabilities based on the logistic function is discussed. Monte Carlo results and empirical findings from the Multiple Risk Factor Intervention Trial indicate that the degree of attenuation of logistic parameter estimates is well approximated by the reliability coefficient when the errors are assumed to be normal random variates and event probabilities are small. In the design of intervention studies, measurement error and temporal variability of risk factors do not usually influence estimates of the probability of developing the disease in the control group, but can result in mis‐estimation of the probability of developing the disease in the experimental group, substantially reducing the statistical power of the clinical trial.
Bioequivalence revisited
Authors: Sheiner Lewis B. 
Published: April 2007

AbstractThe FDA permits marketing of a generic formulation of a drug G for the same indications as a standard preparation S if one can show that G is bioequivalent to S. Present implementation requires convincing evidence that the population mean difference in bioavailability (drug exposure) between the two preparations lies within specified bounds. The basis for this standard does not appear to involve a comprehensive model for the dose‐response relationship, or consideration of clinical issues, notably (i) whether a patient is to commence on the drug or to switch from an established regimen to a new one; or (ii) that the risk of inequivalence relates to uncertainty of outcome. In this paper, I propose a comprehensive model for dose response and a tentative model for risk that addresses these issues. Specifically, I propose two new measures of bioequivalence which are based on these models, which differ in the two clinical circumstances above, and which respond to both bias and variance of outcome. I present two examples, and some simulations of the application of the new measures.
Molecular Exchange through Membranes of Poly(2‐vinylpyridine‐block‐ethylene oxide) Vesicles
Authors: Leson Alina , Hauschild Stephan , Rank Anja , Neub Andreas , Schubert Rolf , Förster Stephan , Mayer Christian 
Published: April 2007

AbstractThe molecular exchange of tracer molecules through the membranes of dispersed vesicles of the block copolymer poly(2‐vinylpyridine‐block‐ethylene oxide) was studied by using NMR spectroscopy combined with pulsed field gradients. The hydrodynamic radius of the tracer molecules was varied systematically to obtain a permeability profile of the vesicle membrane. In addition, the effect of system parameters, such as temperature, pH value, vesicle size, and thickness of the vesicle membrane, was studied. In the case of rapid exchange with average residence times significantly smaller than 10 s, the permeation is observed under equilibrium conditions and the data are analyzed by using a simple analytical approach. For slow exchange processes with average residence times above 10 s, the permeation is monitored in a time‐resolved measurement under nonequilibrium conditions. Generally, the transmembrane exchange rate of the tracer clearly depends on its hydrodynamic radius. The characteristics of this dependence indicate the presence of two different mechanisms of membrane penetration, one dominating for smaller and one for larger tracer molecules, respectively. The exchange rate also shows a significant dependence on the bilayer thickness and on the vesicle diameter. By contrast, no variation of the membrane permeability with the temperature or the pH value could be detected as long as the vesicles remain stable.
Seismic analysis, design, and review for tall buildings
Authors: Moehle Jack P. 
Published: April 2007

AbstractWhereas current building codes legally apply to seismic design of tall buildings, their prescriptive provisions do not adequately address many critical aspects. Performance‐based engineering provides a desirable alternative. Application of performance‐based procedures requires: an understanding of the relation between performance and nonlinear modeling; selection and manipulation of ground motions appropriate to design hazard levels; selection of appropriate nonlinear models and analysis procedures; interpretation of results to determine design quantities based on nonlinear dynamic analysis procedures; careful attention to structural details; and peer review by independent qualified experts to help assure the building official that the proposed materials and system are acceptable. These topics are discussed with an emphasis on tall buildings. Copyright © 2006 John Wiley & Sons, Ltd.
Idealized backbone model for existing reinforced concrete columns and comparisons with FEMA 356 criteria
Authors: Elwood Kenneth J. , Moehle Jack P. 
Published: April 2007

AbstractAn idealized backbone model defining the key damage states of an existing reinforced concrete column (flexural yielding, shear failure, and axial load failure) is developed using drift capacity models. Assumptions and limitations of the drift capacity models are discussed. To demonstrate the application of the idealized backbone model to the evaluation of existing reinforced concrete columns, the model is compared to data from shake table tests and measured drifts from an instrumented building damaged during the Northridge earthquake. The results from the backbone model are also compared with modeling parameters recommended in current seismic rehabilitation guidelines. Copyright © 2006 John Wiley & Sons, Ltd.
Electron‐beam‐pumped light sources using graphite nanoneedle field emitters and Si electron‐transparent films
Authors: Mimura Hidenori , Shiozawa Kazufumi , Neo Yoichiro , Okada Morihiro , Takahasi Masafumi , Hashiguchi Gen 
Published: April 2007

AbstractSputter‐induced carbon nanoneedle field emitters and Si electron‐transparent films have been developed for electron‐beam‐pumped light sources. The sputter‐induced carbon nanoneedle field emitters exhibited a stable electron emission of 0.1 mA at an average field of 13 V/µm. The 1.5‐µm thick Si electron‐transparent films achieved an electron transmittance of about 60% at an acceleration voltage of 27 kV. An electron‐beam‐pumped light source was demonstrated from the excitation of N2 gas, and a N2 gas spectrum was clearly observed. The increase of the beam current is important for increasing the light intensity. Copyright © 2007 Institute of Electrical Engineers of Japan© 2007 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.
Basic characteristics of a space charge balance sensor using a MOSFET
Authors: Sakuyama Masafumi , Takeuchi Manabu , Terashige Takashi , Yoshioka Sayuri , Okano Kazuo 
Published: April 2007

AbstractA new space charge balance sensor was developed in order to control ion generation in an air ionizer. Conventionally, a charged plate monitor is widely used in order to detect the space charge balance of the air ionizer. However, the charged plate monitor is not suitable for the feedback system for several reasons. The proposed sensor, in spite of its very simple structure, can detect the space charge balance instantly with enough sensitivity, with a shorter response time, and has a possibility of higher spatial resolution compared to a charge plate monitor. An air ionizer control system that can generate a specific quantity of ions at desired locations is considered to be easily implemented by using feedback of the sensed signal to the air ionizer. Copyright © 2007 Institute of Electrical Engineers of Japan© 2007 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.