3,910 research outputs found
Estimating Marginal Hazard Ratios by Simultaneously Using A Set of Propensity Score Models: A Multiply Robust Approach
The inverse probability weighted Cox model is frequently used to estimate marginal hazard ratios. Its validity requires a crucial condition that the propensity score model is correctly specified. To provide protection against misspecification of the propensity score model, we propose a weighted estimation method rooted in empirical likelihood theory. The proposed estimator is multiply robust in that it is guaranteed to be consistent when a set of postulated propensity score models contains a correctly specified model. Our simulation studies demonstrate satisfactory finite sample performance of the proposed method in terms of consistency and efficiency. We apply the proposed method to compare the risk of postoperative hospitalization between sleeve gastrectomy and Roux-en-Y gastric bypass using data from a large medical claims and billing database.We further extend the development to multi-site studies to enable each site to postulate multiple site-specific propensity score models
Rejoinder: Matched Pairs and the Future of Cluster-Randomized Experiments
Rejoinder to "The Essential Role of Pair Matching in Cluster-Randomized
Experiments, with Application to the Mexican Universal Health Insurance
Evaluation" [arXiv:0910.3752]Comment: Published in at http://dx.doi.org/10.1214/09-STS274REJ the
Statistical Science (http://www.imstat.org/sts/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Weak Lensing Reconstruction and Power Spectrum Estimation: Minimum Variance Methods
Large-scale structure distorts the images of background galaxies, which
allows one to measure directly the projected distribution of dark matter in the
universe and determine its power spectrum. Here we address the question of how
to extract this information from the observations. We derive minimum variance
estimators for projected density reconstruction and its power spectrum and
apply them to simulated data sets, showing that they give a good agreement with
the theoretical minimum variance expectations. The same estimator can also be
applied to the cluster reconstruction, where it remains a useful reconstruction
technique, although it is no longer optimal for every application. The method
can be generalized to include nonlinear cluster reconstruction and photometric
information on redshifts of background galaxies in the analysis. We also
address the question of how to obtain directly the 3-d power spectrum from the
weak lensing data. We derive a minimum variance quadratic estimator, which
maximizes the likelihood function for the 3-d power spectrum and can be
computed either from the measurements directly or from the 2-d power spectrum.
The estimator correctly propagates the errors and provides a full correlation
matrix of the estimates. It can be generalized to the case where redshift
distribution depends on the galaxy photometric properties, which allows one to
measure both the 3-d power spectrum and its time evolution.Comment: revised version, 36 pages, AAS LateX, submitted to Ap
Exit polling and racial bloc voting: Combining individual-level and RC ecological data
Despite its shortcomings, cross-level or ecological inference remains a
necessary part of some areas of quantitative inference, including in United
States voting rights litigation. Ecological inference suffers from a lack of
identification that, most agree, is best addressed by incorporating
individual-level data into the model. In this paper we test the limits of such
an incorporation by attempting it in the context of drawing inferences about
racial voting patterns using a combination of an exit poll and precinct-level
ecological data; accurate information about racial voting patterns is needed to
assess triggers in voting rights laws that can determine the composition of
United States legislative bodies. Specifically, we extend and study a hybrid
model that addresses two-way tables of arbitrary dimension. We apply the hybrid
model to an exit poll we administered in the City of Boston in 2008. Using the
resulting data as well as simulation, we compare the performance of a pure
ecological estimator, pure survey estimators using various sampling schemes and
our hybrid. We conclude that the hybrid estimator offers substantial benefits
by enabling substantive inferences about voting patterns not practicably
available without its use.Comment: Published in at http://dx.doi.org/10.1214/10-AOAS353 the Annals of
Applied Statistics (http://www.imstat.org/aoas/) by the Institute of
Mathematical Statistics (http://www.imstat.org
A Partially Reflecting Random Walk on Spheres Algorithm for Electrical Impedance Tomography
In this work, we develop a probabilistic estimator for the voltage-to-current
map arising in electrical impedance tomography. This novel so-called partially
reflecting random walk on spheres estimator enables Monte Carlo methods to
compute the voltage-to-current map in an embarrassingly parallel manner, which
is an important issue with regard to the corresponding inverse problem. Our
method uses the well-known random walk on spheres algorithm inside subdomains
where the diffusion coefficient is constant and employs replacement techniques
motivated by finite difference discretization to deal with both mixed boundary
conditions and interface transmission conditions. We analyze the global bias
and the variance of the new estimator both theoretically and experimentally. In
a second step, the variance is considerably reduced via a novel control variate
conditional sampling technique
Estimating the polarization degree of polarimetric images in coherent illumination using maximum likelihood methods
This paper addresses the problem of estimating the polarization degree of polarimetric images in coherent illumination. It has been recently shown that the degree of polarization associated to polarimetric images can be estimated by the method of moments applied to two or four images assuming fully developed speckle. This paper shows that the estimation can also be conducted by using maximum likelihood methods. The maximum likelihood estimators of the
polarization degree are derived from the joint distribution of the image intensities. We show that the joint distribution of polarimetric images is a multivariate gamma distribution whose marginals are univariate, bivariate or trivariate gamma distributions. This property is used to derive maximum likelihood estimators of the polarization degree using two, three or four images. The proposed
estimators provide better performance that the estimators of
moments. These results are illustrated by estimations conducted on synthetic and real images
Finite Dimensional Statistical Inference
In this paper, we derive the explicit series expansion of the eigenvalue
distribution of various models, namely the case of non-central Wishart
distributions, as well as correlated zero mean Wishart distributions. The tools
used extend those of the free probability framework, which have been quite
successful for high dimensional statistical inference (when the size of the
matrices tends to infinity), also known as free deconvolution. This
contribution focuses on the finite Gaussian case and proposes algorithmic
methods to compute the moments. Cases where asymptotic results fail to apply
are also discussed.Comment: 14 pages, 13 figures. Submitted to IEEE Transactions on Information
Theor
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