ISSN:0162-1459    

DOI:10.1080/01621459.1993.10476287

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

I propose a multiple time series model for data from a network of monitoring stations that have both temporal and spatial correlation. The model includes a separate mean and trend for each monitoring station and obtains spatial estimates of mean and trend by smoothing the observed values over a rectangular grid using a discrete smoothing prior. Smoothing parameters and covariance estimates can be chosen subjectively or selected using indirect generalized cross-validation. The gridded values and their standard errors can be used for several purposes, including inference on regional means or trends and improving monitoring networks via station rearrangement.

ISSN:0162-1459    

DOI:10.1080/01621459.1993.10476288

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Nonparametric maximum likelihood estimation of the probability of failing from a particular cause by timetin the presence of other acting causes (i.e., the cause-specific failure probability) is discussed. A commonly used incorrect approach is to take 1 minus the Kaplan-Meier (KM) estimator (1 – KM), whereby patients who fail of extraneous causes are treated as censored observations. Examples showing the extent of bias in using the 1-KM approach are presented using clinical oncology data. This bias can be quite large if the data are uncensored or if a large percentage of patients fail from extraneous causes prior to the occurrence of failures from the cause of interest. Each cause-specific failure probability is mathematically defined as a function of all of the cause-specific hazards. Therefore, nonparametric estimates of the cause-specific failure probabilities may not be able to identify categorized covariate effects on the cause-specific hazards. These effects would be correctly identified by cause-specific cumulative hazard or KM plots in which the extraneous causes of failure are treated as censored observations. Examples are provided. Finally, nonparametric graphical representation of the two distinct cause-specific failure components of the mixture model (i.e., the probability of ever failing from a particular cause and the time-to-failure distribution given that a patient will fail of that cause) are presented. The difficulty in extrapolating the nonparametric estimates beyond the range of observed failure times is highlighted. In addition, the mathematical relationship of a single covariatezand the two cause-specific failure components is shown for the case wherezacts multiplicatively on the cause-specific hazards. Examples are considered using the important prognostic factors in adult soft tissue sarcoma (STS) of the extremity and adult acute lymphoblastic leukemia (ALL). In each of the mathematical and actual examples considered, the characteristic that was associated with a higher hazard rate of failure from the disease was also associated with a higher probability of ever failing from the disease as well as a shorter time-to-failure distribution given that failure due to the disease will occur. Thus it may be quite common in chronic disease situations to find prognosticators that are associated with both cause-specific failure components.

ISSN:0162-1459    

DOI:10.1080/01621459.1993.10476289

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

One approach to estimating the number of cells with high growth potential in a cancer patient is the assessment of tumor cell colony formation in semisolid medium. A potential indicator of the clinical response to a specific drug is the capacity of the drug to reduce the formation of cell colonies. Often, a Poisson log-linear model provides an adequate representation of the dose-response curve. This model is then summarized by the dose required to reduce the number of colonies to a predetermined percentage of the maximum growth. But more general models are needed to account for the overdispersion and the resistant subpopulations often present in these assays. This article explores alternative methods for selecting the dose levels to obtain precise estimates of the parameters of interest. Optimal dose allocations for a single patient or a group of patients are derived. Methods to incorporate the information from pilot studies are discussed. A study of drug sensitivity with leukemia patients provides the framework and motivation for the problem.

ISSN:0162-1459    

DOI:10.1080/01621459.1993.10476290

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

This article uses latent structure analysis to model ordered category ratings by multiple experts on the appropriateness of indications for the medical procedure carotid endarterectomy. The statistical method used is a form of located latent class analysis, which combines elements of latent class and latent trait analysis. It assumes that treatment indications fall into distinct latent classes, with each latent class corresponding to a different level of appropriateness. The appropriateness rating of a treatment indication by a rater is assumed determined by the latent class membership of the indication, rating category thresholds of the rater, and random measurement error. The located latent class model has two alternative forms: a normal ogive form, which derives from the assumption of normally distributed measurement error, and a logistic approximation to the normal form. The approach has the following advantages for the analysis of ordered category ratings by multiple experts: (1) it assesses whether different raters base ratings on the same or different criteria; (2) it assesses rater bias—the tendency of some raters to make higher or lower ratings than others; (3) it characterizes rater differences in rating category definitions; (4) it provides theoretically based methods for combining the ratings of different raters; and (5) it provides a description of the distribution of the latent trait. The data examined are appropriateness ratings on 848 indications for carotid endarterectomy made by nine medical experts. The located latent class approach provides unique insights concerning the data. It identifies what appears to be a set of clear nonindications for carotid endarterectomy, but a corresponding set of clear indications is not evident. The results indicate that all raters measured a common latent trait of treatment appropriateness, but that some measured the trait better than others. Rater differences in overall bias and rating category definitions are evident. Two methods are used to combine raters' ratings. One uses ratings to calculate a continuous appropriateness score for each indication. The other uses ratings to assign indications to discrete outcome categories, each corresponding to a specific level of appropriateness. The located latent class approach for ordered category measures has possible applications besides the analysis of expert ratings, such as item analysis. Potential extensions of the model are discussed.

ISSN:0162-1459    

DOI:10.1080/01621459.1993.10476291

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

We apply a sequential Bayesian sampling procedure to study two models of learning in repeated games. In the first model individuals learn only about an opponent when they play her or him repeatedly but do not update from their experience with that opponent when they move on to play the same game with other opponents. We label this thenonsequential model. In the second model individuals use Bayesian updating to learn about population parameters from each of their opponents, as well as learning about the idiosyncrasies of that particular opponent. We call this thesequential model. We sequentially sample observations on the behavior of experimental subjects in the so-called “centipede game.” This game allows for a trade-off between competition and cooperation, which is of interest in many economic situations. At each point in time, the “state” of our dynamic problem consists of our beliefs about the two models and beliefs about the nuisance parameters of the two models. Our “choice” set is to sample or not to sample one more data point and, if we should not sample, which of the models to select. After 19 matches (4 subjects per match), we stop and reject the nonsequential model in favor of the sequential model.

ISSN:0162-1459    

DOI:10.1080/01621459.1993.10476292

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Defects on the surface of railroad tracks have been the cause of growing concern over the past three decades. The automated detection and classification of rail surface defects would be of great assistance to rail maintenance planners, who develop grinding strategies to prevent the development of potentially dangerous deterioration. Videotaped images of the surface of rail have been obtained, but they are subject to distortions due to the acquisition process as well as physical phenomena on the track itself. In this analysis, an algorithm is presented for the simultaneous restoration and segmentation of objects in a two-dimensional image. The algorithm relies on distributions that model the relationships between sites and neighbors in order to restore a distorted image to an estimate of its ideal form, and also obtain detailed information about the objects located in the image. The foundation of the algorithm is the Iterated Conditional Modes procedure for image restoration. The resulting extension is capable of providing detailed measurements of the geometric features of objects detected in an image. The extended algorithm is applied to an image distorted by simulated noise, and also to an image taken from a videotape of a rail surface. The results of the analysis demonstrate the potential for accurate detection, measurement, and classification of rail surface defects.

ISSN:0162-1459    

DOI:10.1080/01621459.1993.10476293

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

A firm in the early stages of financial distress exhibits characteristics different from those of healthy firms. As the economic condition of a firm worsens, its financial characteristics shift toward those of failed firms. Practitioners in the financial sector have long been interested in the early detection of a firm's slide toward insolvency. Several models have been developed with this purpose in mind, but these older models are static in nature. Therefore, a need exists for the development of business failure prediction models that assess the financial condition of firms sequentially over time. This article addresses this need by presenting a sequential business failure prediction model.

ISSN:0162-1459    

DOI:10.1080/01621459.1993.10476294

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Numerical integration of a multimodal integrandf(θ) is approached by Monte Carlo integration via importance sampling. A mixture of multivariatetdensity functions is suggested as an importance functiong(θ), for its easy random variate generation, thick tails, and high flexibility. The number of components in the mixture is determined by the number of modes off(θ), and the mixing weights and location and scale parameters of the component distributions are determined by numerical minimization of a Monte Carlo estimate of the squared variation coefficient of the weight functionf(θ)/g(θ). Stratified importance sampling and control variates are shown to be particularly effective variance reduction techniques in this case. The algorithm is applied to a 10-dimensional example and shown to yield significant improvement over usual integration schemes.

ISSN:0162-1459    

DOI:10.1080/01621459.1993.10476295

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

A tree-based method for censored survival data is developed, based on maximizing the difference in survival between groups of patients represented by nodes in a binary tree. The method includes a pruning algorithm with optimal properties analogous to the classification and regression tree (CART) pruning algorithm. Uniform convergence of the estimates of the conditional cumulative hazard and survival functions is discussed, and an example is given to show the utility of the algorithm for developing prognostic classifications for patients.

ISSN:0162-1459    

DOI:10.1080/01621459.1993.10476296

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor