857 research outputs found
Hydride generation using a metallic atomizer after microwave-assisted extraction for inorganic arsenic speciation in biological samples
AbstractThe present speciation method reports the determination of inorganic arsenic forms, using metallic furnace hydride generation atomic absorption spectrometry. The inorganic As speciation is carried out using mild conditions for hydride formation, such as slightly acid pH media (4.50) and low tetrahydridoborate(1−) concentration (0.1% (w/v)). Limits of detection and quantification of 2.0 and 6.6μgL−1 of iAs(III) are obtained using optimized conditions. Additionally, microwave-assisted extraction using water as solvent is carried out to provide the appropriate environment for As species extraction as well as impeding inter-conversion between species. With these analytical strategies, As was accurately determined (at 99.9% confidence level) in water and plankton samples
Using the Bootstrap to test for symmetry under unknown dependence
This paper considers tests for symmetry of the one-dimensional marginal distribution of fractionally integrated processes. The tests are implemented by using an autoregressive sieve bootstrap approximation to the null sampling distribution of the relevant test statistics. The sieve bootstrap allows inference on symmetry to be carried out without knowledge of either the memory parameter of the data or of the appropriate norming factor for the test statistic and its asymptotic distribution. The small-sample properties of the proposed method are examined by means of Monte Carlo experiments, and applications to real-world data are also presented
Frequentist and Bayesian measures of confidence via multiscale bootstrap for testing three regions
A new computation method of frequentist -values and Bayesian posterior
probabilities based on the bootstrap probability is discussed for the
multivariate normal model with unknown expectation parameter vector. The null
hypothesis is represented as an arbitrary-shaped region. We introduce new
parametric models for the scaling-law of bootstrap probability so that the
multiscale bootstrap method, which was designed for one-sided test, can also
computes confidence measures of two-sided test, extending applicability to a
wider class of hypotheses. Parameter estimation is improved by the two-step
multiscale bootstrap and also by including higher-order terms. Model selection
is important not only as a motivating application of our method, but also as an
essential ingredient in the method. A compromise between frequentist and
Bayesian is attempted by showing that the Bayesian posterior probability with
an noninformative prior is interpreted as a frequentist -value of
``zero-sided'' test
How to best sample a periodic probability distribution, or on the accuracy of Hamiltonian finding strategies
Projective measurements of a single two-level quantum mechanical system (a
qubit) evolving under a time-independent Hamiltonian produce a probability
distribution that is periodic in the evolution time. The period of this
distribution is an important parameter in the Hamiltonian. Here, we explore how
to design experiments so as to minimize error in the estimation of this
parameter. While it has been shown that useful results may be obtained by
minimizing the risk incurred by each experiment, such an approach is
computationally intractable in general. Here, we motivate and derive heuristic
strategies for experiment design that enjoy the same exponential scaling as
fully optimized strategies. We then discuss generalizations to the case of
finite relaxation times, T_2 < \infty.Comment: 7 pages, 2 figures, 3 appendices; Quantum Information Processing,
Online First, 20 April 201
Markov basis and Groebner basis of Segre-Veronese configuration for testing independence in group-wise selections
We consider testing independence in group-wise selections with some
restrictions on combinations of choices. We present models for frequency data
of selections for which it is easy to perform conditional tests by Markov chain
Monte Carlo (MCMC) methods. When the restrictions on the combinations can be
described in terms of a Segre-Veronese configuration, an explicit form of a
Gr\"obner basis consisting of moves of degree two is readily available for
performing a Markov chain. We illustrate our setting with the National Center
Test for university entrance examinations in Japan. We also apply our method to
testing independence hypotheses involving genotypes at more than one locus or
haplotypes of alleles on the same chromosome.Comment: 25 pages, 5 figure
Factor copula models for item response data
Factor or conditional independence models based on copulas are proposed for multivariate discrete data such as item responses. The factor copula models have interpretations of latent maxima/minima (in comparison with latent means) and can lead to more probability in the joint upper or lower tail compared with factor models based on the discretized multivariate normal distribution (or multidimensional normal ogive model). Details on maximum likelihood estimation of parameters for the factor copula model are given, as well as analysis of the behavior of the log-likelihood. Our general methodology is illustrated with several item response data sets, and it is shown that there is a substantial improvement on existing models both conceptually and in fit to data
Comparison of structural transformations and superconductivity in compressed Sulfur and Selenium
Density-functional calculations are presented for high-pressure structural
phases of S and Se. The structural phase diagrams, phonon spectra,
electron-phonon coupling, and superconducting properties of the isovalent
elements are compared. We find that with increasing pressure, Se adopts a
sequence of ever more closely packed structures (beta-Po, bcc, fcc), while S
favors more open structures (beta-Po, simple cubic, bcc). These differences are
shown to be attributable to differences in the S and Se core states. All the
compressed phases of S and Se considered are calculated to have weak to
moderate electron-phonon coupling strengths consistent with superconducting
transition temperatures in the range of 1 to 20 K. Our results compare well
with experimental data on the beta-Po --> bcc transition pressure in Se and on
the superconducting transition temperature in beta-Po S. Further experiments
are suggested to search for the other structural phases predicted at higher
pressures and to test theoretical results on the electron-phonon interaction
and superconducting properties
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Relationships Between Archimedean Copulas and Morgenstern Utility Functions
The (additive) generator of an Archimedean copula is a strictly decreasing and convexfunction, while Morgenstern utility functions (applying to risk aversion decision makers) arenondecreasing and concave. In this presentation, relationships between generators and utilityfunctions are established. For some well known Archimedean copula families, links betweenthe generator and the corresponding utility function are demonstrated.Some new copulafamilies are derived from classes of utility functions which appeared in the literature, andtheir properties are discussed. It is shown how dependence properties of an Archimedeancopula translate into properties of the utility function from whichthey are constructed
Towards Machine Wald
The past century has seen a steady increase in the need of estimating and
predicting complex systems and making (possibly critical) decisions with
limited information. Although computers have made possible the numerical
evaluation of sophisticated statistical models, these models are still designed
\emph{by humans} because there is currently no known recipe or algorithm for
dividing the design of a statistical model into a sequence of arithmetic
operations. Indeed enabling computers to \emph{think} as \emph{humans} have the
ability to do when faced with uncertainty is challenging in several major ways:
(1) Finding optimal statistical models remains to be formulated as a well posed
problem when information on the system of interest is incomplete and comes in
the form of a complex combination of sample data, partial knowledge of
constitutive relations and a limited description of the distribution of input
random variables. (2) The space of admissible scenarios along with the space of
relevant information, assumptions, and/or beliefs, tend to be infinite
dimensional, whereas calculus on a computer is necessarily discrete and finite.
With this purpose, this paper explores the foundations of a rigorous framework
for the scientific computation of optimal statistical estimators/models and
reviews their connections with Decision Theory, Machine Learning, Bayesian
Inference, Stochastic Optimization, Robust Optimization, Optimal Uncertainty
Quantification and Information Based Complexity.Comment: 37 page
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