1,721,134 research outputs found
Recovering Natural History: Modeling Cardiovascular Biomarkers in the Presence of Endogenous Medication Use
No full textThesis (Ph.D.)--University of Washington, 2016-08In the modern era, cardiovascular biomarkers are often measured in the presence of medication use, whereby the observed value is different than the underlying untreated value for participants on medication. However, for certain problems, the natural history of the biomarker that would have occurred in the absence of medication use is of greater interest than the observed value. In observational data, medication use is nonrandom in that participants on medication tend to have higher underlying biomarker values than participants off medication. That is to say that medication use is endogenous. When faced with endogenous medication use, traditional methods such as adjustment for medication use in linear regression models are inappropriate. The goal of this dissertation is to develop methods to estimate associations between predictors of interest and biomarker outcomes in the presence of endogenous medication use. First, we focus on methods for use in a cross-sectional setting. Heckman's treatment effects model, as suggested by its name, has historically been used to estimate the effect of medication use on a continuous outcome. In this research, we take a definitive departure from the historical use of the model, in that we utilize the Heckman framework in order to estimate associations between exposures and underlying (off-medication) outcomes, regarding the effect of medication on the biomarker as a nuisance rather than a parameter of interest. We show that the treatment effects model is fairly robust to departures from several of its main assumptions. One assumption to which the treatment effects model is particularly sensitive, however, is the assumption of uniform treatment effects. In particular, the expected effect of medication use on the biomarker is presumed to be constant across participants (an assumption that is often thought to be unrealistic in practice). We extend the treatment effects model to allow effect modification, or "subgroup-specific" treatment effects. The second major aim of this dissertation pertains to developing methodology to address endogenous medication use when repeated measures are available on subjects over time. Very little work has been done to address the challenges of endogenous medication use in longitudinal data. For certain types of probit analyses, existing methods invoke standard results on M-estimation theory to construct asymptotically valid estimates of marginal parameters. As cardiovascular biomarkers of interest show strong within-subject correlation over time, there is much efficiency to be gained by modeling that correlation. We seek to understand situations in which accounting for correlation can be advantageous (e.g., in the setting of deterministic covariates), and elucidate efficiency gains with specification of a working covariance. These two objectives primarily target bias reduction for the challenge of addressing endogenous medication use in estimating biomarker associations. Improving estimation of these associations can help us better understand underlying biological mechanisms of disease and better motivate future clinical research
Covariate-specific Breakpoints in Personalized Evaluation of Left Ventricular Enlargement
No full textThesis (Master's)--University of Washington, 2020In this thesis, we outline a flexible and interpretable iterative method to fit generalized segmented linear models with covariate-specific breakpoints and evaluate its statistical performance by conducting simulation studies under several data-generating settings. We then apply our methodology to data from the Multi-Ethnic Study of Atherosclerosis (MESA) Study and investigate breakpoints in the association between cardiovascular outcome measures (ASCVD score and 10-year all-cause mortality) and left ventricular mass by gender and BMI. Our results suggest that the estimator is root n consistent. As expected, we also found that there is a positive correlation between BMI and breakpoints of left ventricular mass, after which the risk of mortality considerably increases. In addition, for a given BMI, the breakpoints appear to differ by sex, with males having much higher values. The method allows estimation of an LV enlargement threshold that differs by body size and gender
Risk Prediction In Cardiovascular Epidemiology: Considerations for Modeling Composite Endpoints
No full textThesis (Master's)--University of Washington, 2016-08This thesis reviews construction and evaluation of risk prediction scores in the context of cardiovascular disease. We give an overview of the clinical guidelines that make use of cardiovascular disease risk scores as well as current practices in recalibration of risk scores outside of the development population. Because cardiovascular disease is a composite of heart attack and stroke, we summarize approaches to modeling and recalibrating composite endpoints and give our major reservations about the clinical utility of such a composite endpoint and the ability to recalibrate such an endpoint. A method of predicting individual outcomes is proposed that is roughly statistically equivalent to the current practice in terms of calibration and discrimination but increases useful information for clinicians in that, for a given composite risk, risks for the component outcomes can vary greatly. Knowledge of these component risks should influence clinical recommendations. Limitations are discussed and next steps are suggested, largely in terms of simulating competing risks survival data and comparing the performance of recalibration methods for composite endpoint whose component risk models differ substantially
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Variations on the Author
Get PDF“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
Get PDFWe 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
Get PDFWe 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
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
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