4,547 research outputs found
Multiplicative random walk Metropolis-Hastings on the real line
In this article we propose multiplication based random walk Metropolis
Hastings (MH) algorithm on the real line. We call it the random dive MH (RDMH)
algorithm. This algorithm, even if simple to apply, was not studied earlier in
Markov chain Monte Carlo literature. The associated kernel is shown to have
standard properties like irreducibility, aperiodicity and Harris recurrence
under some mild assumptions. These ensure basic convergence (ergodicity) of the
kernel. Further the kernel is shown to be geometric ergodic for a large class
of target densities on . This class even contains realistic target
densities for which random walk or Langevin MH are not geometrically ergodic.
Three simulation studies are given to demonstrate the mixing property and
superiority of RDMH to standard MH algorithms on real line. A share-price
return data is also analyzed and the results are compared with those available
in the literature
Modelling and simulation framework for reactive transport of organic contaminants in bed-sediments using a pure java object - oriented paradigm
Numerical modelling and simulation of organic contaminant reactive transport in the environment is being increasingly
relied upon for a wide range of tasks associated with risk-based decision-making, such as prediction of contaminant
profiles, optimisation of remediation methods, and monitoring of changes resulting from an implemented remediation
scheme. The lack of integration of multiple mechanistic models to a single modelling framework, however, has
prevented the field of reactive transport modelling in bed-sediments from developing a cohesive understanding of
contaminant fate and behaviour in the aquatic sediment environment. This paper will investigate the problems involved
in the model integration process, discuss modelling and software development approaches, and present preliminary
results from use of CORETRANS, a predictive modelling framework that simulates 1-dimensional organic contaminant
reaction and transport in bed-sediments
Noisy Monte Carlo: Convergence of Markov chains with approximate transition kernels
Monte Carlo algorithms often aim to draw from a distribution by
simulating a Markov chain with transition kernel such that is
invariant under . However, there are many situations for which it is
impractical or impossible to draw from the transition kernel . For instance,
this is the case with massive datasets, where is it prohibitively expensive to
calculate the likelihood and is also the case for intractable likelihood models
arising from, for example, Gibbs random fields, such as those found in spatial
statistics and network analysis. A natural approach in these cases is to
replace by an approximation . Using theory from the stability of
Markov chains we explore a variety of situations where it is possible to
quantify how 'close' the chain given by the transition kernel is to
the chain given by . We apply these results to several examples from spatial
statistics and network analysis.Comment: This version: results extended to non-uniformly ergodic Markov chain
An appraisal of retained placentae in Ibadan: a five year review
Objectives: To determine the frequency of retained placenta at the University College Hospital Ibadan (UCH). and to describe the socio-demographic characteristics of the patients and examine the risk factors predisposing to retained placenta.Methods: This is a descriptive study covering a period of 5 years from January 1st 2002 to December 31st 2006. During the study period, 4980 deliveries took place at the University College Hospital, Ibadan and 106 cases of retained placenta were managed making the incidence 2.13 per cent of all births.Results: During the five year period, there were 106 patients with retained placenta; of these, 90 (84.9%) case notes were available for analysis. The mean age was 29.37 ± 4.99 years. First and second Para accounted for 52 per cent of the patients. Majority of the patient were unbooked for antenatal care in UCH with booked patients accounting for 27.8 per cent of the cases. The mean gestational age at delivery was 34.29 ± 6.02. Three patients presented to the hospital in shock of which 2 died on account ofsevere haemorrhagic shock. Fifty-eight patients (64.8%) presented with anaemia (packed cell volume less than 30 per cent) and 35 patients (38.8%) had blood transfusion ranging between 1-4 pints. 1 patient required hysterectomy on account of morbidly adherent placenta. Eleven patients (12.2%) had placenta retention in the past, 28 patients (31%) had a previous dilatation and curettage, 14 patients (15.5%) had previous caesarean sections and 47 patients (41.3%) had no known predisposing factorsConclusion: Retained placenta still remains a potentially life threatening condition in the tropics due to the associated haemorrhage, and other complications related to its removal. The incidence and severity may be decreased by health education, women empowerment and the provision of facilities for essential obstetric services by high skilled health care providers in ensuring a properly conducted delivery with active management of the third stage of labour
Exact simulation of diffusions
We describe a new, surprisingly simple algorithm, that simulates exact sample paths of a class of stochastic differential equations. It involves rejection sampling and, when applicable, returns the location of the path at a random collection of time instances. The path can then be completed without further reference to the dynamics of the target process
Sampling constrained probability distributions using Spherical Augmentation
Statistical models with constrained probability distributions are abundant in
machine learning. Some examples include regression models with norm constraints
(e.g., Lasso), probit, many copula models, and latent Dirichlet allocation
(LDA). Bayesian inference involving probability distributions confined to
constrained domains could be quite challenging for commonly used sampling
algorithms. In this paper, we propose a novel augmentation technique that
handles a wide range of constraints by mapping the constrained domain to a
sphere in the augmented space. By moving freely on the surface of this sphere,
sampling algorithms handle constraints implicitly and generate proposals that
remain within boundaries when mapped back to the original space. Our proposed
method, called {Spherical Augmentation}, provides a mathematically natural and
computationally efficient framework for sampling from constrained probability
distributions. We show the advantages of our method over state-of-the-art
sampling algorithms, such as exact Hamiltonian Monte Carlo, using several
examples including truncated Gaussian distributions, Bayesian Lasso, Bayesian
bridge regression, reconstruction of quantized stationary Gaussian process, and
LDA for topic modeling.Comment: 41 pages, 13 figure
The size-Ramsey number of powers of paths
Given graphs and and a positive integer , say that
\emph{is -Ramsey for} , denoted
, if every -colouring of the edges of
contains a monochromatic copy of . The \emph{size-Ramsey number} \sr(H) of
a graph is defined to be
\sr(H)=\min\{|E(G)|\colon G\rightarrow (H)_2\}. Answering a
question of Conlon, we prove that, for every fixed~, we have
\sr(P_n^k)=O(n), where~ is the th power of the
-vertex path (i.e., the graph with vertex set and
all edges such that the distance between and in
is at most ). Our proof is probabilistic, but can also be made constructive.Most of the work for this paper was done during my PhD, which was half funded by EPSRC grant reference 1360036, and half by Merton College Oxford.
The third author was partially supported by FAPESP
(Proc.~2013/03447-6) and by CNPq (Proc.~459335/2014-6,
310974/2013-5). The fifth author was
supported by FAPESP (Proc.~2013/11431-2, Proc.~2013/03447-6 and
Proc.~2018/04876-1) and partially by CNPq (Proc.~459335/2014-6).
This research was supported in part by CAPES (Finance Code 001).
The collaboration of part of the authors was supported by a
CAPES/DAAD PROBRAL grant (Proc.~430/15)
Annular pigment band on the posterior capsule following blunt ocular trauma: a case report
BACKGROUND: To report an unusual case of annular pigment band on the posterior capsule following blunt ocular trauma. CASE PRESENTATION: We describe an annular pigment band on the posterior capsule following blunt ocular trauma in a 28-year old male patient. Repeat examinations revealed no evidence of other signs of blunt ocular trauma or pigment dispersion syndrome in either eye. CONCLUSION: The annular pigment band in this case corresponds to the adherence of the hyaloideocapsulare ligament to the posterior capsule and reconfirms its rare visualization in the living eye. This finding may be an isolated sign of blunt ocular trauma and a compromised integrity of the vitreolenticular interface should be strongly suspected. We recommend careful documentation in context of future cataract surgery in these eyes
On the flexibility of the design of Multiple Try Metropolis schemes
The Multiple Try Metropolis (MTM) method is a generalization of the classical
Metropolis-Hastings algorithm in which the next state of the chain is chosen
among a set of samples, according to normalized weights. In the literature,
several extensions have been proposed. In this work, we show and remark upon
the flexibility of the design of MTM-type methods, fulfilling the detailed
balance condition. We discuss several possibilities and show different
numerical results
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