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Search Results: 1 - 10 of 467901 matches for " Alicia A. Johnson "
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Geometric Ergodicity & Scanning Strategies For Two-Component Gibbs Samplers
Alicia A. Johnson,Owen Burbank
Statistics , 2012,
Abstract: In any Markov chain Monte Carlo analysis, rapid convergence of the chain to its target probability distribution is of practical and theoretical importance. A chain that converges at a geometric rate is geometrically ergodic. In this paper, we explore geometric ergodicity for two-component Gibbs samplers which, under a chosen scanning strategy, evolve by combining one-at-a-time updates of the two components. We compare convergence behaviors between and within three such strategies: composition, random sequence scan, and random scan. Our main results are twofold. First, we establish that if the Gibbs sampler is geometrically ergodic under any one of these strategies, so too are the others. Further, we establish a simple and verifiable set of sufficient conditions for the geometric ergodicity of the Gibbs samplers. Our results are illustrated using two examples.
Gibbs Sampling for a Bayesian Hierarchical General Linear Model
Alicia A. Johnson,Galin L. Jones
Mathematics , 2007,
Abstract: We consider a Bayesian hierarchical version of the normal theory general linear model which is practically relevant in the sense that it is general enough to have many applications and it is not straightforward to sample directly from the corresponding posterior distribution. Thus we study a block Gibbs sampler that has the posterior as its invariant distribution. In particular, we establish that the Gibbs sampler converges at a geometric rate. This allows us to establish conditions for a central limit theorem for the ergodic averages used to estimate features of the posterior. Geometric ergodicity is also a key component for using batch means methods to consistently estimate the variance of the asymptotic normal distribution. Together, our results give practitioners the tools to be as confident in inferences based on the observations from the Gibbs sampler as they would be with inferences based on random samples from the posterior. Our theoretical results are illustrated with an application to data on the cost of health plans issued by health maintenance organizations.
Comment: Gibbs Sampling, Exponential Families, and Orthogonal Polynomials
Galin L. Jones,Alicia A. Johnson
Statistics , 2008, DOI: 10.1214/08-STS252C
Abstract: Comment on ``Gibbs Sampling, Exponential Families, and Orthogonal Polynomials'' [arXiv:0808.3852]
A Modified Gibbs Sampler on General State Spaces
Alicia A. Johnson,James M. Flegal
Statistics , 2013,
Abstract: We present a modified Gibbs sampler for general state spaces. We establish that this modification can lead to substantial gains in statistical efficiency while maintaining the overall quality of convergence. We illustrate our results in two examples including a toy Normal-Normal model and a Bayesian version of the random effects model.
Component-Wise Markov Chain Monte Carlo: Uniform and Geometric Ergodicity under Mixing and Composition
Alicia A. Johnson,Galin L. Jones,Ronald C. Neath
Statistics , 2009, DOI: 10.1214/13-STS423
Abstract: It is common practice in Markov chain Monte Carlo to update the simulation one variable (or sub-block of variables) at a time, rather than conduct a single full-dimensional update. When it is possible to draw from each full-conditional distribution associated with the target this is just a Gibbs sampler. Often at least one of the Gibbs updates is replaced with a Metropolis-Hastings step, yielding a Metropolis-Hastings-within-Gibbs algorithm. Strategies for combining component-wise updates include composition, random sequence and random scans. While these strategies can ease MCMC implementation and produce superior empirical performance compared to full-dimensional updates, the theoretical convergence properties of the associated Markov chains have received limited attention. We present conditions under which some component-wise Markov chains converge to the stationary distribution at a geometric rate. We pay particular attention to the connections between the convergence rates of the various component-wise strategies. This is important since it ensures the existence of tools that an MCMC practitioner can use to be as confident in the simulation results as if they were based on independent and identically distributed samples. We illustrate our results in two examples including a hierarchical linear mixed model and one involving maximum likelihood estimation for mixed models.
Low usage of government healthcare facilities for acute respiratory infections in guatemala: implications for influenza surveillance
Kim A Lindblade, April J Johnson, Wences Arvelo, Xingyou Zhang, Hannah T Jordan, Lissette Reyes, Alicia M Fry, Norma Padilla
BMC Public Health , 2011, DOI: 10.1186/1471-2458-11-885
Abstract: We used a stratified, two-stage cluster survey sample to select 1200 households from the Department of Santa Rosa. Trained interviewers screened household residents for self-reported pneumonia in the last year and influenza-like illness (ILI) in the last month and asked about healthcare utilization for each illness episode.We surveyed 1131 (94%) households and 5449 residents between October and December 2006 and identified 323 (6%) cases of pneumonia and 628 (13%) cases of ILI. Treatment for pneumonia outside the home was sought by 92% of the children <5 years old and 73% of the persons aged five years and older. For both children <5 years old (53%) and persons aged five years and older (31%) who reported pneumonia, private clinics were the most frequently reported source of care. For ILI, treatment was sought outside the home by 81% of children <5 years old and 65% of persons aged five years and older. Government ambulatory clinics were the most frequently sought source of care for ILI both for children <5 years old (41%) and persons aged five years and older (36%).Sentinel surveillance for influenza and other respiratory infections based in government health facilities in Guatemala will significantly underestimate the burden of disease. Adjustment for healthcare utilization practices will permit more accurate estimation of the incidence of influenza and other respiratory pathogens in the community.As the 2009 influenza A (H1N1) pandemic highlighted, surveillance for influenza is now a worldwide priority.[1,2] At the 58th World Assembly in 2005, The World Health Organization adopted a resolution calling for Member States to fortify and coordinate national strategies to prepare for an influenza pandemic, including establishment of surveillance systems for human influenza.[3] To assist with the development of standardized influenza surveillance systems in the Americas, the Pan American Health Organization (PAHO) and the United States Centers for Disease Control and P
Set up to Fail: Inadequate Educational Support for Orphans in Central Kenya  [PDF]
Ginger A. Johnson
Advances in Anthropology (AA) , 2011, DOI: 10.4236/aa.2011.11001
Abstract: In response to Kenya's goal of free and universal primary education for every child by 2015, this paper describes a few of the obstacles that one of the most visible periphery populations in Kenya, orphaned children, face in attempting to reach this objective. The most frequently cited barriers of children and their caretakers to consistent school attendance included: inability to pay school fees, lack of a school uniform, difficulty in providing assis- tance to orphaned children, presence of disease/illness in the family and disruption of education due to political violence. Conducted in a Kikuyu community in the Kinangop District of Central Kenya following the 2007/2008 presidential election riots, this study utilized multiple regression, logistic regression and MANOVA statistical tests to determine if families caring for orphaned children of primary school age differed significantly from families with no orphans in the home. Discriminant function and Mahalanobis testing further revealed differ- ences in types of households, with the presence of orphans in the home (particularly AIDS orphans) significantly increasing the amount of school fees owed per family. Qualitative data obtained from semi-structured interviews with families and open-ended interviews with their primary school aged children contextualized study results and inform policy recommendations.
Trophic Status of Shallow Lakes of La Pampa (Argentina) and Its Relation with the Land Use in the Basin and Nutrient Internal Load  [PDF]
Santiago A. Echaniz, Alicia M. Vignatti
Journal of Environmental Protection (JEP) , 2013, DOI: 10.4236/jep.2013.411A007

Phosphorus and nitrogen are essential nutrients for living organisms. Their concentration in the water of an aquatic ecosystem is one of the factors responsible for the trophic status of the lake and is related to the soils of the region and to the human activities carried out in their basins. These nutrients are also found in the bottom sediments, where they can either be retained or re-enter the water column. Since the information about the concentrations of nutrients in the water of some lakes of La Pampa (Argentina) is fragmentary, the aim of this study is to describe the trophic status of some shallow lakes of the semiarid center of Argentina and analyze its relation with the human activities in their basins, the concentrations of nutrients and organic matter and particle size distribution of sediments. To this end, we studied ten shallow lakes subjected to different anthropogenic influences (agriculture, agriculture and livestock and impacted by cities). All were hypertrophic and the concentrations of total phosphorus and total nitrogen were among the highest reported globally. Since some lakes had no fish, cladoceran grazing (top-down effect) led them have reduced concentrations of phytoplankton chlorophyll-a and high water transparency. This relativizes the use of these parameters to determine the trophic status. The sediments of seven of the studied lakes were predominated by fine sands, whereas three were predominated by silts. Nutrient and organic matter content were high, with higher concentrations in lakes with prevalence of fine particles. The reduced adsorption capacity of sediments, the resuspension by wind, the anthropogenic input and the accumulation favored by the arheic character of the basins would explain the high concentrations of nutrients in the water of these Pampean environments.

Wind-Solar Hybrid Electrical Power Production to Support National Grid: Case Study - Jordan  [PDF]
Ghassan HALASA, Johnson A. ASUMADU
Energy and Power Engineering (EPE) , 2009, DOI: 10.4236/epe.2009.12011
Abstract: The paper presents the next generation of power energy systems using solar- and wind-energy systems for the country of Jordan. Presently with the oil prices are on the rise, the cost of electrical power production is very high. The opportunity of a large wind and solar hybrid power production is being explored. Sights are chosen to produce electricity using the wind in the Mountains in Northern Jordan and the sun in the Eastern Desert. It is found that the cost of windmill farm to produce 100 - 150 MW is US$290 million while solar power station to produce 100 MW costs US$560 million. The electrical power costs US$0.02/kWh for the wind power and US$0.077 for the solar power. The feasibility for using wind and solar energies is now when the price oil reaches US$ 100.00 per barrel. The paper also discusses different power electronics circuits and control methods to link the renewable energy to the national grid. This paper also looks at some of the modern power electronics converters and electrical generators, which have improved significantly solar and wind energy technologies.
Hair It Is: Examining the Experiences of Black Women with Natural Hair  [PDF]
Tabora A. Johnson, Teiahsha Bankhead
Open Journal of Social Sciences (JSS) , 2014, DOI: 10.4236/jss.2014.21010
Abstract: Who am I and how do I feel about who I am, are essential questions that help define and construct identity. For Black women and girls, identity is inextricably linked to their relationship to and presentation of their hair. Our research presents findings from an Internet based survey conducted with 529 Black women exploring their experiences when wearing their hair in its natural state (not thermally or chemically straightened). These are preliminary findings from the study with reference to the composition of the study participants and how they responded to key questions related to how they perceived when wearing their hair naturally.
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