In many clinical studies, subjects are at risk of experiencing more than one type of potentially recurrent event. In some
situations, however, the occurrence of an event is observed, but the specific type is not determined. We consider the analysis
of this type of incomplete data when the objectives are to summarize features of conditional intensity functions and associated
treatment effects, and to study the association between different types of event. Here we describe a likelihood approach based
on joint models for the multi-type recurrent events where parameter estimation is obtained from a Monte-Carlo EM algorithm.
Simulation studies show that the proposed method gives unbiased estimators for regression coefficients and variance–covariance
parameters, and the coverage probabilities of confidence intervals for regression coefficients are close to the nominal level.
When the distribution of the frailty variable is misspecified, the method still provides estimators of the regression coefficients
with good properties. The proposed method is applied to a motivating data set from an asthma study in which exacerbations
were to be sub-typed by cellular analysis of sputum samples as eosinophilic or non-eosinophilic. 相似文献
AbstractMarkov processes offer a useful basis for modeling the progression of organisms through successive stages of their life cycle. When organisms are examined intermittently in developmental studies, likelihoods can be constructed based on the resulting panel data in terms of transition probability functions. In some settings however, organisms cannot be tracked individually due to a difficulty in identifying distinct individuals, and in such cases aggregate counts of the number of organisms in different stages of development are recorded at successive time points. We consider the setting in which such aggregate counts are available for each of a number of tanks in a developmental study. We develop methods which accommodate clustering of the transition rates within tanks using a marginal modeling approach followed by robust variance estimation, and through use of a random effects model. Composite likelihood is proposed as a basis of inference in both settings. An extension which incorporates mortality is also discussed. The proposed methods are shown to perform well in empirical studies and are applied in an illustrative example on the growth of the Arabidopsis thaliana plant. 相似文献
Many chronic diseases feature recurring clinically important events. In addition, however, there often exists a random variable which is realized upon the occurrence of each event reflecting the severity of the event, a cost associated with it, or possibly a short term response indicating the effect of a therapeutic intervention. We describe a novel model for a marked point process which incorporates a dependence between continuous marks and the event process through the use of a copula function. The copula formulation ensures that event times can be modeled by any intensity function for point processes, and any multivariate model can be specified for the continuous marks. The relative efficiency of joint versus separate analyses of the event times and the marks is examined through simulation under random censoring. An application to data from a recent trial in transfusion medicine is given for illustration. 相似文献
Since 1982, the elderly poverty rate reported by the U.S. Census Bureau has fallen below the rate for the nonelderly population. This is cited as evidence of the success of U.S. social policies to benefit the elderly. But lower elderly poverty rates are an artifact of the fact that a lower, more stringent poverty line is applied to the elderly living in one- and two-person households, who constitute 85% of elderly persons. If the same poverty standard is applied to the elderly as to the nonelderly, the poverty rates are the same or slightly higher. The poverty line was originally based on the cost of an adequate diet. The lower standard for the elderly was based on the fact that the elderly consume fewer calories than nonelderly adults. This article shows there is no justification for this lower standard, and recommends its elimination. The overall nutrient requirements of the elderly are not lower, and the elderly spend a higher proportion of their budgets on food and on other necessities (shelter, health care) than the nonelderly. Alternative units of analysis examined under different income-pooling assumptions also show that poverty rates are not lower among the elderly than the nonelderly. 相似文献
Parameter reduction can enable otherwise infeasible design and uncertainty studies with modern computational science models that contain several input parameters. In statistical regression, techniques for sufficient dimension reduction (SDR) use data to reduce the predictor dimension of a regression problem. A computational scientist hoping to use SDR for parameter reduction encounters a problem: a computer prediction is best represented by a deterministic function of the inputs, so data comprised of computer simulation queries fail to satisfy the SDR assumptions. To address this problem, we interpret SDR methods sliced inverse regression (SIR) and sliced average variance estimation (SAVE) as estimating the directions of a ridge function, which is a composition of a low-dimensional linear transformation with a nonlinear function. Within this interpretation, SIR and SAVE estimate matrices of integrals whose column spaces are contained in the ridge directions’ span; we analyze and numerically verify convergence of these column spaces as the number of computer model queries increases. Moreover, we show example functions that are not ridge functions but whose inverse conditional moment matrices are low-rank. Consequently, the computational scientist should beware when using SIR and SAVE for parameter reduction, since SIR and SAVE may mistakenly suggest that truly important directions are unimportant.
Mental health problems in the college population appear to be increasing in number and severity. University counseling services are limited in the number of counselors they can hire to meet the increasing demands. In addition, it is believed that many students do not actually seek much-needed counseling services due to lack of knowledge about mental health problems or services, stigma, or denial of the severity of the problem. Left unrecognized and untreated, mental health problems may lead to students dropping out or failing out of college, attempting or committing suicide, or engaging in other risky, dangerous behaviors that may result in serious injury, disability, or death. Mental health nursing faculty can help address this problem on college campuses by offering courses on mental health issues and skills, becoming involved with NAMI-on-Campus groups, and seeking grant opportunities that can be used to help develop and enhance services for the college community. Information about how to establish a NAMI-on-Campus group can be obtained by accessing http://www.namioncampus.org or http://www.nami.org. 相似文献