ISSN:0162-1459    

DOI:10.1080/01621459.1990.10475300

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

One of the main goals of modern crystallography is the determination of the detailed internal structure of crystalline matter, at the atomic level. Statistical analyses and, in particular, random-walk models play a central role in inferring structural information from crystallographic data. Such methods are routinely employed by crystallographers in the determination of crystal symmetry from the experimental data, and in the solution of the outstandingly important problem for this discipline, the phase problem. Three classes of approaches are discussed: (a) methods based entirely on the central limit theorem; (b) approximate expansions in terms of orthogonal polynomials that have the central-limit-theorem pdf as their weight function—that is, Gram–Charlier and Edgeworth expansions; and (c) pdf's that are exactly formulated and reduced to computable forms, represented as Fourier and Fourier–Bessel series. Both univariate and multivariate pdf's of crystallographic interest are derived and discussed. Some other approximate probabilistic approaches that have been applied to crystallographic problems are also briefly reviewed.

ISSN:0162-1459    

DOI:10.1080/01621459.1990.10475301

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

From a cancer registry in the San Francisco Bay area we obtained survival data for 2,495 women diagnosed with breast cancer at ages 55–64. We relate mortality among these women to the time since diagnosis and to the stage of the disease at diagnosis. We divide the study period, extending through 10 years, into five two-year periods, and for each stage we assume a constant hazard rate during each of these periods. Let λ = (λjk) be theJ×Kmatrix of hazard rates for theJ= 5 periods andK= 5 stages. The most general model allows λjkto vary freely. A plot of maximum likelihood estimates of the hazard rates shows some tendency for increase with stage, but no simple patterns or parallelism across stage. We seek more restrictive models, to get simpler interpretations. The exponential model assumes that although λjkmay vary with stage, it is constant over the five periods for each stage. This model, which assumes no dependence of hazard rate on time since diagnosis, is quite restrictive, and indeed the likelihood ratio test of the exponential versus the general model rejects it strongly. Not quite as restrictive as the exponential model is a proportional-hazards model, which assumes that the log-hazard rates for the first four stages are parallel. Nevertheless, the likelihood ratio test of this model versus the general model rejects it as well. We explore the possibility that one of the more restrictive models is appropriate but that the bad fit is due to errors in staging. To do so, we replace the aforementioned models with ones that accommodate stage misclassification. Using the EM algorithm to compute maximum likelihood estimates and likelihood ratio statistics, we find that the exponential model is again rejected, but that the proportional-hazards model fits the data. This example shows that simple models with straightforward interpretations might be discarded needlessly if covariate misclassifications are ignored. Simulations support this possibility. When data are generated according to a proportional-hazards model with stage misclassifications, ignoring the misclassification can result in missing the proportional-hazards model. Simulations revealed other points. In particular, large samples are needed to detect classification errors. In addition, misclassification models give hazard-rate estimates that can be much more variable than those of models without misclassification.

ISSN:0162-1459    

DOI:10.1080/01621459.1990.10475302

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

This article describes new methods for estimating survival distributions based on nonparametric curve estimators. One approach improves the estimation of long-term survival rates. Simulation studies using Weibull and lognormal data show that even in the case found to be least favorable, the new method has less than one-seventh the prediction error of all conventional life-table (LT) or Kaplan–Meier (KM) estimators, even when the LT and KM techniques are optimized for the purpose of long-term survival estimation. In addition to conventional survival applications, one can also estimate the probability of being disease-free at different ages and following different exposures to possibly harmful environmental contaminants. This approach is particularly useful in situations where the effects of a confounding, nuisance, or effect-modifying variable cannot be confidently modeled in a parametric form. The new techniques are based on a closed-form nonparametric maximum likelihood curve estimator expressed in terms of separate curve estimates obtained from samples of randomly censored and uncensored times to failure—that is, subsurvival populations.

ISSN:0162-1459    

DOI:10.1080/01621459.1990.10475303

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

National statistical offices meet an increasing demand for the dissemination of microdata. This trend can be at variance with the care taken not to disclose data about individuals. Therefore, the risk of disclosure of each microdata set to be released should be assessed. If this risk is too high, measures have to be taken to protect the data set. This article describes the disclosure problem, and explains why it is a real problem. Using the concept of uniqueness, some theory is developed that can help establish the risk of identification. It turns out that useful microdata sets can only be released if some of the disclosure risks are dealt with by legal arrangements, rather than by restrictions on the data to be released.

ISSN:0162-1459    

DOI:10.1080/01621459.1990.10475304

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

One notion of language development suggests that infants' and toddlers' vocalizations are produced at random with no particular patterns or organization. In this article two tests (the pairs test and the gap test) that are used to check the randomness properties of computer-generated pseudorandom numbers are modified to study the sounds produced by infants 13 months of age and toddlers 24 months of age. Deviations from the expected patterns that would be observed if the children did produce sound segments at random suggest features of the development of language skills. The information provided by the lack of fit of the model corresponding to the pairs test is used to develop models that capture the common patterns in transitions among sound classifications in children's early speech.

ISSN:0162-1459    

DOI:10.1080/01621459.1990.10475305

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Two models, the U.S. census model and the latent-class model, are compared in their application to evaluating measurements of ethnicity. Although the census approach assumes that the response categories of a questionnaire item correspond to groups in the population, the latent-class approach seeks to assess whether any set of response categories can represent observed ethnic heterogeneity. Data collected using the 1990 census Hispanic-origin question and other instruments for measuring ethnicity suggest that the latent-class approach is superior whenever the response categories are not known to be valid. In particular, using the latent-class model, this article rejects the census model's assumption of a single dimension of meaning underlying responses to the Hispanic-origin question.

ISSN:0162-1459    

DOI:10.1080/01621459.1990.10475306

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

This article compares several promising data-driven methods for selecting the bandwidth of a kernel density estimator. The methods compared are least squares cross-validation, biased cross-validation, and a plug-in rule. The comparison is done by asymptotic rate of convergence to the optimum and a simulation study. It is seen that the plug-in bandwidth is usually most efficient when the underlying density is sufficiently smooth, but is less robust when there is not enough smoothness present. We believe the plug-in rule is the best of those currently available, but there is still room for improvement.

ISSN:0162-1459    

DOI:10.1080/01621459.1990.10475307

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The use of the rank transform for testing problems in some two-factor designs is considered. The rank transform procedure consists of replacing the observations by their ranks in the combined sample and performing one of the standard analysis of variance (ANOVA) procedures on these ranks. The asymptotic version of the rank transform is introduced, and its usefulness as a means for understanding the nature of the rank transform is examined. It is demonstrated that the asymptotic version of the transformation helps identify the testing problems where the rank transform method works and helps suggest which ANOVA procedure should be used on the ranks. This approach is applied on the balanced and unbalanced nested model and the balanced and unbalanced two-way layout with and without interaction. The extension of the rank transform method to some of these models is new. The proposed procedures do not share the known simplicity of existing rank transform statistics, but they do allow heteroscedasticity in the errors. The asymptotic power of the procedures is derived, and comparisons with aligned rank andFtests are made.

ISSN:0162-1459    

DOI:10.1080/01621459.1990.10475308

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

This article concerns computational methods for the bootstrap that are more efficient than the straightforward Monte Carlo methods usually used. The bootstrap is considered in its simplest form: in a one-sample nonparametric problem, where the goal is to estimate the bias or variance of some statistic by bootstrap sampling, or to set approximate confidence intervals for a parameter of interest in terms of various percentiles of the bootstrap distribution. The methods of this article can, in favorable situations, reduce the necessary number of bootstrap replications manyfold. Moreover, simple diagnostics are available to see whether or not any particular case is accessible to these methods.

ISSN:0162-1459    

DOI:10.1080/01621459.1990.10475309

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor