ISSN:0162-1459    

DOI:10.1080/01621459.1995.10476482

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

In a model, such as a logit regression model, we wish to compare the relative importance of two groups of predictors for various objectives. In the example that motivated this work, the model predicts patient outcomes during hospital stays, and we wish to measure the relative contribution of patient and hospital characteristics to the variation in outcomes among patients and among hospitals. This is done using the relative dispersion of patient and hospital contributions to the fitted outcomes. As is seen, this question is distinct from other common questions, including the quality of the overall fit, the degree to which the outcome is accurately predicted, the statistical significance of groups of predictors, and the correlations among and between groups of predictors. Relevant point estimates, confidence intervals, and hypothesis tests are developed. In the example, we examine three outcome measures and find that nearly all of the predictable variation in patient outcomes and most of the predictable variation in outcomes among hospitals reflects variation in patient characteristics rather than hospital characteristics; however, this is true in varying degrees for the three outcomes.

ISSN:0162-1459    

DOI:10.1080/01621459.1995.10476483

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Any screening program based on a quantitative risk factor has the objective of identifying individuals at high risk of disease. When the risk factor varies within individuals over time or is subject to measurement error, repeat measurements should aid risk assessment. This article presents statistical guidelines for sequential screening based on measurements accumulated over time. The aim for each subject is to decide whether (a) risk is sufficiently low that screening can be stopped, or (b) risk is sufficiently high that intervention is indicated, or (c) further measurements should be obtained to improve risk assessment. The guidelines presented are optimal in a population, public health sense; that is, they are designed to give the maximal expected risk in the group identified for intervention, subject to constraints on that group's size and on the cost of screening (defined in terms of the average number of measurements per person required). An example concerning screening for cardiovascular disease in middle-aged men using diastolic blood pressure measurements is presented.

ISSN:0162-1459    

DOI:10.1080/01621459.1995.10476484

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

A question that has received a great deal of attention in evaluating new treatments in acquired immune deficiency syndrome (AIDS) clinical trials is that of finding a good surrogate marker for clinical progression. The identification of such a marker may be useful in assessing the efficacy of new therapies in a shorter period. The number of CD4-lymphocyte counts has been proposed as such a potential marker for human immune virus (HIV) trials because of its observed correlation with clinical outcome. But to evaluate the role of CD4 counts as a potential surrogate marker, we must better understand the relationship of clinical outcome to an individual's CD4 count history over time. The Cox proportional hazards regression model is used to study the relationship between CD4 counts as a time-dependent covariate and survival. Because the CD4 counts are measured only periodically and with substantial measurement error and biological variation, standard methods for estimating the parameters in the Cox model by maximizing the partial likelihood are no longer appropriate. Instead, we propose a two-stage approach. In the first stage the longitudinal CD4 count data are modeled using a repeated measures random components model. In the second stage methods for estimating the parameters in a Cox model when the data are assumed to be of this form are derived. We also considered methods to account for missing data patterns. These methods are applied to CD4 data from a randomized clinical trial of AIDS patients where half of the patients were randomized to receive Zidovudine (ZDV) and the other half were randomized to receive a placebo. Although a strong correlation between CD4 counts and survival is demonstrated, we also show that CD4 counts may not serve as a useful surrogate marker for assessing treatments for this population of patients.

ISSN:0162-1459    

DOI:10.1080/01621459.1995.10476485

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Intratumor DNA heterogeneity—as assessed by flow-cytometric cellular DNA analysis—is predictive of prognosis in many types of cancer. But some truly heterogeneous tumors apparently have a substantial chance of being misclassified as normal. One way to enhance the assay's sensitivity is to test multiple samples from each tumor specimen. This article describes an analysis of data from a study of endometrial cancer, with a view to establishing the number of samples needed to test to reliably assess heterogeneity. The analysis involves selection of a model for the data and estimation of test sensitivity and predictive value. We analyze the data from both Bayesian and frequentist perspectives. The data suggest that the degree of within-tumor correlation is relatively high; consequently, there is little to be gained from taking multiple samples per tumor.

ISSN:0162-1459    

DOI:10.1080/01621459.1995.10476486

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

It may be clinically useful to know when prostate-specific antigen (PSA) levels first begin to rise rapidly and to determine if the natural history of PSA progression is different in men with locally confined prostate cancers compared to men with metastatic tumors. This article uses a nonlinear mixed-effects model to describe longitudinal changes in PSA in men before their prostate cancers were detected clinically. Repeated measurements of PSA are available for 18 subjects with a diagnosis of prostate cancer based on prostate biopsy. PSA measurements were determined on repeated frozen serum samples collected from subjects with at least 10.0 years and up to 25.6 years of observation before the cancer was detected. A piecewise model is used to describe this data. The model is linear long before the cancer was detected and exponential nearer the time the cancer was detected. The time at which the PSA levels change from linear to exponential PSA progression is unknown but can be estimated by including random terms that allow each subject to have his own transition time. The model also accounts for two groups of patients—those with local or regional cancer and those with advanced cancer or whose cancer has metastasized. Various parameters are allowed to differ between these two groups. By backward elimination of statistically nonsignificant parameters, a model is found that adequately describes the data. The model represents a situation where local/regional and advanced/metastatic cancers have similar rates of PSA progression, but advanced/metastatic cancers are diagnosed later. Piecewise mixed-effects models may be useful in a variety of research settings where it is necessary to estimate the unknown time of an event.

ISSN:0162-1459    

DOI:10.1080/01621459.1995.10476487

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

This article develops a survey design where the questionnaire is split into components and individuals are administered the varying subsets of the components. A multiple imputation method for analyzing data from this design is developed, in which the imputations are created by random draws from the posterior predictive distribution of the missing parts, given the observed parts by using Gibbs sampling under a general location scale model. Results from two simulation studies that investigate the properties of the inferences using this design are reported. In the first study several random split questionnaire designs are imposed on the complete data from an existing survey collected using a long questionnaire, and the corresponding data elements are extracted to form split data sets. Inferences obtained using the complete data and the split data are then compared. This comparison suggests that little is lost, at least in the example considered, by administering only parts of the questionnaire to each sampled individual. The second simulation study reports on the investigation of the efficiency of the split questionnaire design and the robustness of the estimates to the distributional assumptions used to create imputations. In this study several complete and split data sets were generated under a variety of distributional assumptions, and the imputations for the split data sets were created assuming the normality of the distributions. The sampling properties of the point and interval estimates of the regression coefficient in a particular logistic regression model using both the complete and split data sets were compared. This comparison suggests that the loss in efficiency of the split questionnaire design decreases as the correlation among the variables that are within different parts increases. The proposed multiple imputation method seems to be sensitive to the skewness and relatively insensitive to the kurtosis, contrary to the assumed normality of the distribution for the observables.

ISSN:0162-1459    

DOI:10.1080/01621459.1995.10476488

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Population projections are widely used in both the public and private sectors for planning, budgeting, and analysis. For these purposes, projections are often needed for small areas such as census tracts, zip code areas or traffic analysis zones. Population size, growth constraints, shifting boundaries, and data availability create special problems for small-area projections, however, and very little is known about their forecasting performance. In this article we evaluate the accuracy and bias of projections of total population and population by age group for census tracts in three counties in Florida. We use data from 1970 and 1980 and several simple extrapolation techniques to produce projections for 1990; we then compare these projections with 1990 census counts and evaluate the differences. For the total sample, we find mean absolute errors of 17%-20% for the three most accurate techniques for projecting total population and find no indication of overall bias. For individual age groups, mean absolute errors range from 20%-29%. We believe that this analysis provides valuable information for demographers, planners, marketers, and others who make extensive use of small-area projections.

ISSN:0162-1459    

DOI:10.1080/01621459.1995.10476489

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

This article describes an exploratory analysis of a record of major El Niño events covering the period 1800–1987. This record contains both the years during which the events occurred and a binary indication of their magnitude. The record is modeled as a marked point process in which the times of the events follow a renewal process and the sequence of binary marks follows a stationary first-order Markov chain.

ISSN:0162-1459    

DOI:10.1080/01621459.1995.10476490

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Linear splines and their tensor products are used to estimate the conditional log-hazard function based on possibly censored, positive response data and one or more covariates. An automatic procedure involving the maximum likelihood method, stepwise addition, stepwise deletion, and the Bayes Information Criterion is used to select the final model. The possible models contain proportional hazards models as a subclass, which makes it possible to diagnose departures from proportionality. Cubic splines and two additional log terms are incorporated into a similar model for the unconditional log-hazard function to allow for greater flexibility in the extreme tails. A user interface based on S is described.

ISSN:0162-1459    

DOI:10.1080/01621459.1995.10476491

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor