1,721,100 research outputs found
A protective estimator for linear regression with nonignorably missing Gaussian outcomes
No full textWe propose a method for estimating the regression parameters in a linear regression model for Gaussian data when the outcome variable is missing for some subjects and missingness is thought to be nonignorable. Throughout, we assume that missingness is restricted to the outcome variable and that the covariates are fully observed. Although maximum likelihood estimation of the regression parameters is possible once joint models for the outcome variable and the nonignorable missing data mechanism have been specified, these models are fundamentally nonidentifiable unless unverifiable modeling assumptions are imposed. In this paper, rather than explicitly modeling the nonignorable missingness mechanism, we consider the use of a ‘protective’ estimator of the regression parameters (Brown, 1990). To implement the proposed method, it is necessary to assume that the outcome variable and one of the covariates have an approximate bivariate normal distribution, conditional on the remaining covariates. In addition, it is assumed that the missing data mechanism is conditionally independent of this covariate, given the outcome variable and the remaining covariates; the latter is referred to as the ‘protective’ assumption. A method of moments approach is used to obtain the protective estimator of the regression parameters; the jackknife (Quenouille, 1956) is used to estimate the variance. The method is illustrated using data on the persistence of maternal smoking from the Six Cities Study of the health effects of air pollution (Ware et al., 1984). The results of a simulation study are presented that examine the magnitude of any finite sample bias. © 2004, Sage Publications. All rights reserved.status: Publishe
A protective estimator for longitudinal binary data subject to non-ignorable non-monotone missingness
No full textIn longitudinal studies missing data are the rule not the exception. We consider the analysis of longitudinal binary data with non-monotone missingness that is thought to be non-ignorable. In this setting a full likelihood approach is complicated algebraically and can be computationally prohibitive when there are many measurement occasions. We propose a 'protective' estimator that assumes that the probability that a response is missing at any occasion depends, in a completely unspecified way, on the value of that variable alone. Relying on this 'protectiveness' assumption, we describe a pseudolikelihood estimator of the regression parameters under non-ignorable missingness, without having to model the missing data mechanism directly. The method proposed is applied to CD4 cell count data from two longitudinal clinical trials of patients infected with the human immunodeficiency virus. Copyright 2005 Royal Statistical Society.
A weighted combination of pseudo-likelihood estimators for longitudinal binary data subject to non-ignorable non-monotone missingness
Full text linkFor longitudinal binary data with non-monotone non-ignorably missing outcomes over time, a full likelihood approach is complicated algebraically, and with many follow-up times, maximum likelihood estimation can be computationally prohibitive. As alternatives, two pseudo-likelihood approaches have been proposed that use minimal parametric assumptions. One formulation requires specification of the marginal distributions of the outcome and missing data mechanism at each time point, but uses an 'independence working assumption,' i.e. an assumption that observations are independent over time. Another method avoids having to estimate the missing data mechanism by formulating a 'protective estimator.' In simulations, these two estimators can be very inefficient, both for estimating time trends in the first case and for estimating both time-varying and time-stationary effects in the second. In this paper, we propose the use of the optimal weighted combination of these two estimators, and in simulations we show that the optimal weighted combination can be much more efficient than either estimator alone. Finally, the proposed method is used to analyze data from two longitudinal clinical trials of HIV-infected patients.sponsorship: The authors are grateful for constructive comments from two reviewers, and for the support provided by the following grants from the US National Institutes of Health: AI 60373, CA 68484, CA69222, CA 74015, CA 70101, GM 29745, MH 054693. Andrea Troxel gratefully acknowledges support from the Columbia University Institute for Scholars at Reid Hall, Paris. Geert Molenberghs gratefully acknowledges financial support from the Belgian Science Policy IAP research network #P6/03. (US National Institutes of Health|AI 60373, US National Institutes of Health|CA 68484, US National Institutes of Health|CA69222, US National Institutes of Health|CA 74015, US National Institutes of Health|CA 70101, US National Institutes of Health|GM 29745, US National Institutes of Health|MH 054693, Columbia University Institute for Scholars at Reid Hall, Paris, Belgian Science Policy IAP|P6/03)status: Publishe
The Impact of Radical Prostatectomy Operative Time on Outcomes and Costs
No full textOBJECTIVE To examine the impact of radical prostatectomy (RP) operative time on outcomes and cost, we performed a population-based assessment of operative time as a predictor of outcomes. Although operative time has been used as a metric to evaluate RP surgeon learning curves, the effect of RP operative times on outcomes remains understudied. ;MATERIALS AND METHODS We used US Surveillance, Epidemiology, and End Results-Medicare linked data to identify 7534 men aged >= 66 years diagnosed with prostate cancer during 2003-2007 who underwent RP for localized prostate cancer through 2009. We categorized RP operative time into quartiles (short, intermediate, long, and very long) and used propensity score analyses to assess its impact on perioperative complications, mortality, length of hospitalization, readmissions, emergency room visits, and costs. ;RESULTS Quartiles ranged from 0 to 172 minutes for short, 173 to 214 minutes for intermediate, 215 to 268 minutes for long, and >= 269 minutes for very long RP operative times. After propensity score adjustment, longer operative time was associated with more surgery-related complications (short, 12.0%; intermediate, 12.3%; long, 14.4%; and very long, 22.8%; P <. 001), longer median (interquartile range) length of stay in days (short, 2 [ 2-3]; intermediate, 2 [ 2-3]; long, 2 [ 1- 3]; and very long, 2 [ 1- 3]; P <. 001), and higher median costs (short, 10,957; long, 11,966; P <. 001). ;CONCLUSION Longer RP operative time is associated with more complications, longer lengths of hospital stay, and higher costs. Increasing operative efficiency may reduce complications, length of stay, and health-care costs.(c) 2014 Elsevier Inc
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
A bivariate pseudo-likelihood for incomplete longitudinal binary data with nonignorable non-monotone missingness
Full text linkFor analyzing longitudinal binary data with nonignorable and nonmonotone missing responses, a full likelihood method is complicated algebraically, and often requires intensive computation, especially when there are many follow-up times. As an alternative, a pseudolikelihood approach has been proposed in the literature under minimal parametric assumptions. This formulation only requires specification of the marginal distributions of the responses and missing data mechanism, and uses an independence working assumption. However, this estimator can be inefficient for estimating both time-varying and time-stationary effects under moderate to strong within-subject associations among repeated responses. In this article, we propose an alternative estimator, based on a bivariate pseudolikelihood, and demonstrate in simulations that the proposed method can be much more efficient than the previous pseudolikelihood obtained under the assumption of independence. We illustrate the method using longitudinal data on CD4 counts from two clinical trials of HIV-infected patients.sponsorship: We are grateful for the support provided by grants from the U.S. National Institutes of Health, and the Natural Sciences and Engineering Research Council of Canada. (U.S. National Institutes of Health, Natural Sciences and Engineering Research Council of Canada)status: Publishe
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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