147 research outputs found
Extrapolation before imputation reduces bias when imputing censored covariates
Modeling symptom progression to identify informative subjects for a new
Huntington's disease clinical trial is problematic since time to diagnosis, a
key covariate, can be heavily censored. Imputation is an appealing strategy
where censored covariates are replaced with their conditional means, but
existing methods saw over 200% bias under heavy censoring. Calculating these
conditional means well requires estimating and then integrating over the
survival function of the censored covariate from the censored value to
infinity. To estimate the survival function flexibly, existing methods use the
semiparametric Cox model with Breslow's estimator, leaving the integrand for
the conditional means (the estimated survival function) undefined beyond the
observed data. The integral is then estimated up to the largest observed
covariate value, and this approximation can cut off the tail of the survival
function and lead to severe bias, particularly under heavy censoring. We
propose a hybrid approach that splices together the semiparametric survival
estimator with a parametric extension, making it possible to approximate the
integral up to infinity. In simulation studies, our proposed approach of
extrapolation then imputation substantially reduces the bias seen with existing
imputation methods, even when the parametric extension was misspecified. We
further demonstrate how imputing with corrected conditional means helps to
prioritize patients for future clinical trials.Comment: 16 pages main text (incl. 2 tables and 3 figures); Supplemental
Materials, R code, and R package available on GitHub (linked in main text
Exploring the validity of the complete case analysis for regression models with a right-censored covariate
Despite its drawbacks, the complete case analysis is commonly used in
regression models with missing covariates. Understanding when implementing
complete cases will lead to consistent parameter estimation is vital before
use. Here, our aim is to demonstrate when a complete case analysis is
appropriate for a nuanced type of missing covariate, the randomly
right-censored covariate. Across the censored covariate literature, different
assumptions are made to ensure a complete case analysis produces a consistent
estimator, which leads to confusion in practice. We make several contributions
to dispel this confusion. First, we summarize the language surrounding the
assumptions that lead to a consistent complete case estimator. Then, we show a
unidirectional hierarchical relationship between these assumptions, which leads
us to one sufficient assumption to consider before using a complete case
analysis. Lastly, we conduct a simulation study to illustrate the performance
of a complete case analysis with a right-censored covariate under different
censoring mechanism assumptions, and we demonstrate its use with a Huntington
disease data example
Combining isotonic regression and EM algorithm to predict genetic risk under monotonicity constraint
In certain genetic studies, clinicians and genetic counselors are interested
in estimating the cumulative risk of a disease for individuals with and without
a rare deleterious mutation. Estimating the cumulative risk is difficult,
however, when the estimates are based on family history data. Often, the
genetic mutation status in many family members is unknown; instead, only
estimated probabilities of a patient having a certain mutation status are
available. Also, ages of disease-onset are subject to right censoring. Existing
methods to estimate the cumulative risk using such family-based data only
provide estimation at individual time points, and are not guaranteed to be
monotonic or nonnegative. In this paper, we develop a novel method that
combines Expectation-Maximization and isotonic regression to estimate the
cumulative risk across the entire support. Our estimator is monotonic,
satisfies self-consistent estimating equations and has high power in detecting
differences between the cumulative risks of different populations. Application
of our estimator to a Parkinson's disease (PD) study provides the age-at-onset
distribution of PD in PARK2 mutation carriers and noncarriers, and reveals a
significant difference between the distribution in compound heterozygous
carriers compared to noncarriers, but not between heterozygous carriers and
noncarriers.Comment: Published in at http://dx.doi.org/10.1214/14-AOAS730 the Annals of
Applied Statistics (http://www.imstat.org/aoas/) by the Institute of
Mathematical Statistics (http://www.imstat.org
The Overlooked Value Of Certificates And Associate's Degrees: What Students Need to Know Before They Go to College
This report examines the labor-market value of associate's degrees and certificate programs, finding that field of study especially influences future earnings for these programs since they are tightly linked with specific occupations. The Overlooked Value of Certificates and Associate's Degrees: What Students Need to Know Before They Go to College also reveals that the combined number of certificates and associate's degrees awarded by colleges is similar to the number of bachelor's degrees awardedâaround 2 million per yearâwith certificates and associate's degrees each accounting for about 1 million
Interview with John P. Howe III, 2023-05-24
https://scholarworks.utrgv.edu/somhi/1035/thumbnail.jp
Interview with Dr. Charles Mullins, John Howe, Dr. James Guckian, Bill Cunningham, Nancy Payne, Anne Shepard, and Bob Shepard, 2023-08-04
https://scholarworks.utrgv.edu/somhi/1060/thumbnail.jp
Mycobacterium tuberculosis Responds to Chloride and pH as Synergistic Cues to the Immune Status of its Host Cell
PubMed ID: 23592993This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited
Plasma Metabolites associated With Cognitive Function across Race/Ethnicities affirming the Importance of Healthy Nutrition
INTRODUCTION: We studied the replication and generalization of previously identified metabolites potentially associated with global cognitive function in multiple race/ethnicities and assessed the contribution of diet to these associations.
METHODS: We tested metabolite-cognitive function associations in U.S.A. Hispanic/Latino adults (n = 2222) from the Community Health Study/ Study of Latinos (HCHS/SOL) and in European (n = 1365) and African (n = 478) Americans from the Atherosclerosis Risk In Communities (ARIC) Study. We applied Mendelian Randomization (MR) analyses to assess causal associations between the metabolites and cognitive function and between Mediterranean diet and cognitive function.
RESULTS: Six metabolites were consistently associated with lower global cognitive function across all studies. Of these, four were sugar-related (e.g., ribitol). MR analyses provided weak evidence for a potential causal effect of ribitol on cognitive function and bi-directional effects of cognitive performance on diet.
DISCUSSION: Several diet-related metabolites were associated with global cognitive function across studies with different race/ethnicities.
HIGHLIGHTS: Metabolites associated with cognitive function in Puerto Rican adults were recently identified. We demonstrate the generalizability of these associations across diverse race/ethnicities. Most identified metabolites are related to sugars. Mendelian Randomization (MR) provides weak evidence for a causal effect of ribitol on cognitive function. Beta-cryptoxanthin and other metabolites highlight the importance of a healthy diet
Using ecological models to assess ecosystem status in support of the European Marine Strategy Framework Directive
© 2015 The Authors. Published by Elsevier Ltd. The European Union's Marine Strategy Framework Directive (MSFD) seeks to achieve, for all European seas, "Good Environmental Status" (GEnS), by 2020. Ecological models are currently one of the strongest approaches used to predicting and understanding the consequences of anthropogenic and climate-driven changes in the natural environment. We assess the most commonly used capabilities of the modelling community to provide information about indicators outlined in the MSFD, particularly on biodiversity, food webs, non-indigenous species and seafloor integrity descriptors. We built a catalogue of models and their derived indicators to assess which models were able to demonstrate: (1) the linkages between indicators and ecosystem structure and function and (2) the impact of pressures on ecosystem state through indicators. Our survey identified 44 ecological models being implemented in Europe, with a high prevalence of those that focus on links between hydrodynamics and biogeochemistry, followed by end-to-end, species distribution/habitat suitability, bio-opt ical (remote sensing) and multispecies models. Approximately 200 indicators could be derived from these models, the majority of which were biomass and physical/hydrological/chemical indicators. Biodiversity and food webs descriptors, with âŒ49% and âŒ43% respectively, were better addressed in the reviewed modelling approaches than the non-indigenous species (0.3%) and sea floor integrity (âŒ8%) descriptors. Out of 12 criteria and 21 MSFD indicators relevant to the abovementioned descriptors, currently only three indicators were not addressed by the 44 models reviewed. Modelling approaches showed also the potential to inform on the complex, integrative ecosystem dimensions while addressing ecosystem fundamental properties, such as interactions between structural components and ecosystems services provided, despite the fact that they are not part of the MSFD indicators set. The cataloguing of models and their derived indicators presented in this study, aim at helping the planning and integration of policies like the MSFD which require the assessment of all European Seas in relation to their ecosystem status and pressures associated and the establishment of environmental targets (through the use of indicators) to achieve GEnS by 2020
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