Helping You Find Full Text Journal Articles

Search Results:

Author: Roderick J Little (46)



View Full Text PDF Listings View primary source full text article PDFs.

Dec
2017

Amyotrophic lateral sclerosis (ALS) has an immune component, but previous human studies have not examined immune changes over time.
To assess peripheral inflammatory markers in participants with ALS and healthy control individuals and to track immune changes in ALS and determine whether these changes correlate with disease progression.
In this longitudinal cohort study, leukocytes were isolated from peripheral blood samples from 35 controls and 119 participants with ALS at the ALS Clinic of the University of Michigan, Ann Arbor, from June 18, 2014, through May 26, 2016.

View Full Text PDF Listings View primary source full text article PDFs.

Jun
2016

The potential impact of missing data on the results of clinical trials has received heightened attention recently. A National Research Council study provides recommendations for limiting missing data in clinical trial design and conduct, and principles for analysis, including the need for sensitivity analyses to assess robustness of findings to alternative assumptions about the missing data. A Food and Drug Administration advisory committee raised missing data as a serious concern in their review of results from the ATLAS ACS 2 TIMI 51 study, a large clinical trial that assessed rivaroxaban for its ability to reduce the risk of cardiovascular death, myocardial infarction or stroke in patients with acute coronary syndrome.

View Full Text PDF Listings View primary source full text article PDFs.

Jul
2016

A case study is presented assessing the impact of missing data on the analysis of daily diary data from a study evaluating the effect of a drug for the treatment of insomnia. The primary analysis averaged daily diary values for each patient into a weekly variable. Following the commonly used approach, missing daily values within a week were ignored provided there was a minimum number of diary reports (i.

View Full Text PDF Listings View primary source full text article PDFs.

View Full Text PDF Listings View primary source full text article PDFs.

Apr
2015

Missing data are an unavoidable problem in clinical trials. Most existing missing data approaches assume the missing data are missing at random. However, the missing at random assumption is often questionable when the real causes of missing data are not well known and cannot be tested from observed data.

View Full Text PDF Listings View primary source full text article PDFs.

Jul
2015

Missing values in predictors are a common problem in survival analysis. In this paper, we review estimation methods for accelerated failure time models with missing predictors, and apply a new method called subsample ignorable likelihood (IL) Little and Zhang (J R Stat Soc 60:591-605, 2011) to this class of models. The approach applies a likelihood-based method to a subsample of observations that are complete on a subset of the covariates, chosen based on assumptions about the missing data mechanism.

View Full Text PDF Listings View primary source full text article PDFs.

Jan
2014

To recommend methodological standards in the prevention and handling of missing data for primary patient-centered outcomes research (PCOR).
We searched National Library of Medicine Bookshelf and Catalog as well as regulatory agencies' and organizations' Web sites in January 2012 for guidance documents that had formal recommendations regarding missing data. We extracted the characteristics of included guidance documents and recommendations.

View Full Text PDF Listings View primary source full text article PDFs.

View Full Text PDF Listings View primary source full text article PDFs.

Dec
2012

This paper develops two Bayesian methods for inference about finite population quantiles of continuous survey variables from unequal probability sampling. The first method estimates cumulative distribution functions of the continuous survey variable by fitting a number of probit penalized spline regression models on the inclusion probabilities. The finite population quantiles are then obtained by inverting the estimated distribution function.

View Full Text PDF Listings View primary source full text article PDFs.

View Full Text PDF Listings View primary source full text article PDFs.

Dec
1969

Gene sequences are routinely used to determine the topologies of unrooted phylogenetic trees, but many of the most important questions in evolution require knowing both the topologies and the roots of trees. However, general algorithms for calculating rooted trees from gene and genomic sequences in the absence of gene paralogs are few. Using the principles of evolutionary parsimony (EP) (Lake JA.

View Full Text PDF Listings View primary source full text article PDFs.

Jan
2012

Covariate measurement error is common in epidemiologic studies. Current methods for correcting measurement error with information from external calibration samples are insufficient to provide valid adjusted inferences. We consider the problem of estimating the regression of an outcome Y on covariates X and Z, where Y and Z are observed, X is unobserved, but a variable W that measures X with error is observed.

View Full Text PDF Listings View primary source full text article PDFs.

Sep
2012

We consider the linear regression of outcome Y on regressors W and Z with some values of W missing, when our main interest is the effect of Z on Y, controlling for W. Three common approaches to regression with missing covariates are (i) complete-case analysis (CC), which discards the incomplete cases, and (ii) ignorable likelihood methods, which base inference on the likelihood based on the observed data, assuming the missing data are missing at random (Rubin, 1976b), and (iii) nonignorable modeling, which posits a joint distribution of the variables and missing data indicators. Another simple practical approach that has not received much theoretical attention is to drop the regressor variables containing missing values from the regression modeling (DV, for drop variables).

View Full Text PDF Listings View primary source full text article PDFs.

Oct
2011

This pragmatic randomized trial evaluated the effectiveness of a tailored educational intervention on oral health behaviors and new untreated carious lesions in low-income African-American children in Detroit, Michigan.
Participating families were recruited in a longitudinal study of the determinants of dental caries in 1021 randomly selected children (0-5 years) and their caregivers. The families were examined at baseline in 2002-2004 (Wave I), 2004-2005 (Wave II) and 2007 (Wave III).

View Full Text PDF Listings View primary source full text article PDFs.

May
2010

Data analysis for randomized trials including multi-treatment arms is often complicated by subjects who do not comply with their treatment assignment. We discuss here methods of estimating treatment efficacy for randomized trials involving multi-treatment arms subject to non-compliance. One treatment effect of interest in the presence of non-compliance is the complier average causal effect (CACE) (Angrist et al.

View Full Text PDF Listings View primary source full text article PDFs.

Dec
2011

In clinical trials, a biomarker (S ) that is measured after randomization and is strongly associated with the true endpoint (T) can often provide information about T and hence the effect of a treatment (Z ) on T. A useful biomarker can be measured earlier than T and cost less than T. In this article, we consider the use of S as an auxiliary variable and examine the information recovery from using S for estimating the treatment effect on T, when S is completely observed and T is partially observed.

View Full Text PDF Listings View primary source full text article PDFs.

Aug
2011

We consider the estimation of the regression of an outcome Y on a covariate X, where X is unobserved, but a variable W that measures X with error is observed. A calibration sample that measures pairs of values of X and W is also available; we consider calibration samples where Y is measured (internal calibration) and not measured (external calibration). One common approach for measurement error correction is Regression Calibration (RC), which substitutes the unknown values of X by predictions from the regression of X on W estimated from the calibration sample.

View Full Text PDF Listings View primary source full text article PDFs.

Jun
2011

In this paper, the authors describe a simple method for making longitudinal comparisons of alternative markers of a subsequent event. The method is based on the aggregate prediction gain from knowing whether or not a marker has occurred at any particular age. An attractive feature of the method is the exact decomposition of the measure into 2 components: 1) discriminatory ability, which is the difference in the mean time to the subsequent event for individuals for whom the marker has and has not occurred, and 2) prevalence factor, which is related to the proportion of individuals who are positive for the marker at a particular age.

View Full Text PDF Listings View primary source full text article PDFs.

May
2011

In longitudinal studies of developmental and disease processes, participants are followed prospectively with intermediate milestones identified as they occur. Frequently, studies enroll participants over a range of ages including ages at which some participants' milestones have already passed. Ages at milestones that occur prior to study entry are left censored if individuals are enrolled in the study or left truncated if they are not.

View Full Text PDF Listings View primary source full text article PDFs.

Nov
2010

This work is motivated by a quantitative Magnetic Resonance Imaging study of the differential tumor/healthy tissue change in contrast uptake induced by radiation. The goal is to determine the time in which there is maximal contrast uptake (a surrogate for permeability) in the tumor relative to healthy tissue. A notable feature of the data is its spatial heterogeneity.

View Full Text PDF Listings View primary source full text article PDFs.

Nov
2010

The Internet provides us with tools (user metrics or paradata) to evaluate how users interact with online interventions. Analysis of these paradata can lead to design improvements.
The objective was to explore the qualities of online participant engagement in an online intervention.

View Full Text PDF Listings View primary source full text article PDFs.

Jul
2010

Disclosure limitation is an important consideration in the release of public use data sets. It is particularly challenging for longitudinal data sets, since information about an individual accumulates with repeated measures over time. Research on disclosure limitation methods for longitudinal data has been very limited.

View Full Text PDF Listings View primary source full text article PDFs.

View Full Text PDF Listings View primary source full text article PDFs.

View Full Text PDF Listings View primary source full text article PDFs.

Jul
2010

Raw data on the relationship between known and measured values of an analyte are collected and analyzed to determine the limit of quantification (LOQ) of an assay. In most LOQ problems, the researcher is given an observed value for the marker of interest if this value is greater than the LOQ, and a missing value (

View Full Text PDF Listings View primary source full text article PDFs.

Jun
2010

We propose a Bayesian Penalized Spline Predictive (BPSP) estimator for a finite population proportion in an unequal probability sampling setting. This new method allows the probabilities of inclusion to be directly incorporated into the estimation of a population proportion, using a probit regression of the binary outcome on the penalized spline of the inclusion probabilities. The posterior predictive distribution of the population proportion is obtained using Gibbs sampling.

View Full Text PDF Listings View primary source full text article PDFs.

Dec
1969

This work is motivated by a quantitative Magnetic Resonance Imaging study of the relative change in tumor vascular permeability during the course of radiation therapy. The differences in tumor and healthy brain tissue physiology and pathology constitute a notable feature of the image data-spatial heterogeneity with respect to its contrast uptake profile (a surrogate for permeability) and radiation induced changes in this profile. To account for these spatial aspects of the data, we employ a Gaussian hidden Markov random field (MRF) model.

View Full Text PDF Listings View primary source full text article PDFs.

Apr
2010

Hot deck imputation is a method for handling missing data in which each missing value is replaced with an observed response from a "similar" unit. Despite being used extensively in practice, the theory is not as well developed as that of other imputation methods. We have found that no consensus exists as to the best way to apply the hot deck and obtain inferences from the completed data set.

View Full Text PDF Listings View primary source full text article PDFs.

Feb
2010

Asthma is a serious problem for low-income preteens living in disadvantaged communities. Among the chronic diseases of childhood and adolescence, asthma has the highest prevalence and related health care use. School-based asthma interventions have proven successful for older and younger students, but results have not been demonstrated for those in middle school.

View Full Text PDF Listings View primary source full text article PDFs.

Jul
2010

The goal of the present study was to quantify the population-based background serum concentrations of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) by using data from the reference population of the 2005 University of Michigan Dioxin Exposure Study (UMDES) and the 2003-2004 National Health and Nutrition Examination Survey (NHANES).
Multiple imputation was used to impute the serum TCDD concentrations below the limit of detection by combining the 2 data sources. The background mean, quartiles, and 95th percentile serum TCDD concentrations were estimated by age and sex by using linear and quantile regressions for complex survey data.

View Full Text PDF Listings View primary source full text article PDFs.

Nov
2009

The objective of this study was to evaluate the existence of cognitive plateaus in some individuals during the course of Alzheimer's disease (AD).
Data came from the historical patient group collected via the Consortium to Establish a Registry for Alzheimer's Disease (CERAD, Duke University, 1988-1996). Data reduction was performed by using principal components analysis to derive a single cognitive measure (F1), followed by application of a novel plateau-searching algorithm to individual patient data, looking for stable periods of 3 years or longer.

View Full Text PDF Listings View primary source full text article PDFs.

Dec
1969

A common strategy for handling item nonresponse in survey sampling is hot deck imputation, where each missing value is replaced with an observed response from a "similar" unit. We discuss here the use of sampling weights in the hot deck. The naive approach is to ignore sample weights in creation of adjustment cells, which effectively imputes the unweighted sample distribution of respondents in an adjustment cell, potentially causing bias.

View Full Text PDF Listings View primary source full text article PDFs.

Jun
2009

Selection models and pattern-mixture models are often used to deal with nonignorable dropout in longitudinal studies. These two classes of models are based on different factorizations of the joint distribution of the outcome process and the dropout process. We consider a new class of models, called mixed-effect hybrid models (MEHMs), where the joint distribution of the outcome process and dropout process is factorized into the marginal distribution of random effects, the dropout process conditional on random effects, and the outcome process conditional on dropout patterns and random effects.

View Full Text PDF Listings View primary source full text article PDFs.

Jun
2009

Meta-analysis of studies with missing data.

Biometrics 2009 Jun 18;65(2):487-96. Epub 2008 May 18.
Ying Yuan, Roderick J A Little
Consider a meta-analysis of studies with varying proportions of patient-level missing data, and assume that each primary study has made certain missing data adjustments so that the reported estimates of treatment effect size and variance are valid. These estimates of treatment effects can be combined across studies by standard meta-analytic methods, employing a random-effects model to account for heterogeneity across studies. However, we note that a meta-analysis based on the standard random-effects model will lead to biased estimates when the attrition rates of primary studies depend on the size of the underlying study-level treatment effect.

View Full Text PDF Listings View primary source full text article PDFs.

Sep
2008

Patient preference may influence intervention effects, but has not been extensively studied. Randomized controlled design (N=1075) assessed outcomes when women (60 years+) were given a choice of two formats of a program to enhance heart disease management.
Randomization to "no choice" or "choice" study arms.

View Full Text PDF Listings View primary source full text article PDFs.

Jun
2009

We consider the analysis of clinical trials that involve randomization to an active treatment (T = 1) or a control treatment (T = 0), when the active treatment is subject to all-or-nothing compliance. We compare three approaches to estimating treatment efficacy in this situation: as-treated analysis, per-protocol analysis, and instrumental variable (IV) estimation, where the treatment effect is estimated using the randomization indicator as an IV. Both model- and method-of-moment based IV estimators are considered.

View Full Text PDF Listings View primary source full text article PDFs.

May
2008

Initial trials of web-based smoking-cessation programs have generally been promising. The active components of these programs, however, are not well understood. This study aimed to (1) identify active psychosocial and communication components of a web-based smoking-cessation intervention and (2) examine the impact of increasing the tailoring depth on smoking cessation.

View Full Text PDF Listings View primary source full text article PDFs.

Dec
2007

In a previous study, we validated a polysomnographic assessment for REM sleep behavior disorder (RBD). The method proved to be reliable but required slow, labor-intensive visual scoring of surface electromyogram (EMG) activity. We therefore developed a computerized metric to assess EMG variance and compared the results to those previously published for visual scoring, bed partner-rated RBD symptom scores, and clinical assessments by sleep medicine specialists.

View Full Text PDF Listings View primary source full text article PDFs.

Jan
2008

Quantitative Magnetic Resonance Imaging (qMRI) provides researchers insight into pathological and physiological alterations of living tissue, with the help of which, researchers hope to predict (local) therapeutic efficacy early and determine optimal treatment schedule. However, the analysis of qMRI has been limited to ad-hoc heuristic methods. Our research provides a powerful statistical framework for image analysis and sheds light on future localized adaptive treatment regimes tailored to the individual's response.

View Full Text PDF Listings View primary source full text article PDFs.

Jan
2008

We consider the analysis of longitudinal data sets that include times of recurrent events, where interest lies in variables that are functions of the number of events and the time intervals between events for each individual, and where some cases have gaps when the information was not recorded. Discarding cases with gaps results in a loss of the recorded information in those cases. Other strategies such as simply splicing together the intervals before and after the gap potentially lead to bias.

View Full Text PDF Listings View primary source full text article PDFs.

Dec
2007

This article concerns item nonresponse adjustment for two-stage cluster samples. Specifically, we focus on two types of nonignorable nonresponse: nonresponse depending on covariates and underlying cluster characteristics, and depending on covariates and the missing outcome. In these circumstances, standard weighting and imputation adjustments are liable to be biased.

View Full Text PDF Listings View primary source full text article PDFs.

Dec
1969

We wanted to identify what factors promote career development in patient-oriented clinical research (POCR).
We used a survey questionnaire covering areas relevant to the training of subspecialty fellows and the career development of POCR faculty.
Pursuit of an academic career after fellowship correlated with completion of a clinical project, availability of a clinical research training program, opportunity to earn academic credit, faculty mentorship, and obtaining independent career development funding.

View Full Text PDF Listings View primary source full text article PDFs.

Sep
2004

Noncompliance is a common problem in experiments involving randomized assignment of treatments, and standard analyses based on intention-to-treat or treatment received have limitations. An attractive alternative is to estimate the Complier-Average Causal Effect (CACE), which is the average treatment effect for the subpopulation of subjects who would comply under either treatment (Angrist, Imbens, and Rubin, 1996, Journal of American Statistical Association 91, 444-472). We propose an extended general location model to estimate the CACE from data with noncompliance and missing data in the outcome and in baseline covariates.

View Full Text PDF Listings View primary source full text article PDFs.

Apr
2004

Serum reproductive hormone concentrations were measured longitudinally in a community-based, multiethnic population of midlife women to assess whether ethnic differences exist in the patterns of change in estradiol (E2) and FSH and, if so, whether these differences are explained by host characteristics. We studied 3257 participants from seven clinical sites in the Study of Women's Health Across the Nation (SWAN) who were aged 42-52 yr at baseline and self-identified as African American (28.2%), Caucasian (47.

View Full Text PDF Listings View primary source full text article PDFs.

May
2003

A basic estimation strategy in sample surveys is to weight units inversely proportional to the probability of selection and response. Response weights in this method are usually estimated by the inverse of the sample-weighted response rate in an adjustment cell, that is, the ratio of the sum of the sampling weights of respondents in a cell to the sum of the sampling weights for respondents and non-respondents in that cell. We show by simulations that weighting the response rates by the sampling weights to adjust for design variables is either incorrect or unnecessary.

View Full Text PDF Listings View primary source full text article PDFs.

Back to top