15,347 research outputs found
Efficient Estimation of Approximate Factor Models via Regularized Maximum Likelihood
We study the estimation of a high dimensional approximate factor model in the
presence of both cross sectional dependence and heteroskedasticity. The
classical method of principal components analysis (PCA) does not efficiently
estimate the factor loadings or common factors because it essentially treats
the idiosyncratic error to be homoskedastic and cross sectionally uncorrelated.
For efficient estimation it is essential to estimate a large error covariance
matrix. We assume the model to be conditionally sparse, and propose two
approaches to estimating the common factors and factor loadings; both are based
on maximizing a Gaussian quasi-likelihood and involve regularizing a large
covariance sparse matrix. In the first approach the factor loadings and the
error covariance are estimated separately while in the second approach they are
estimated jointly. Extensive asymptotic analysis has been carried out. In
particular, we develop the inferential theory for the two-step estimation.
Because the proposed approaches take into account the large error covariance
matrix, they produce more efficient estimators than the classical PCA methods
or methods based on a strict factor model
Statistical Inferences Using Large Estimated Covariances for Panel Data and Factor Models
While most of the convergence results in the literature on high dimensional
covariance matrix are concerned about the accuracy of estimating the covariance
matrix (and precision matrix), relatively less is known about the effect of
estimating large covariances on statistical inferences. We study two important
models: factor analysis and panel data model with interactive effects, and
focus on the statistical inference and estimation efficiency of structural
parameters based on large covariance estimators. For efficient estimation, both
models call for a weighted principle components (WPC), which relies on a high
dimensional weight matrix. This paper derives an efficient and feasible WPC
using the covariance matrix estimator of Fan et al. (2013). However, we
demonstrate that existing results on large covariance estimation based on
absolute convergence are not suitable for statistical inferences of the
structural parameters. What is needed is some weighted consistency and the
associated rate of convergence, which are obtained in this paper. Finally, the
proposed method is applied to the US divorce rate data. We find that the
efficient WPC identifies the significant effects of divorce-law reforms on the
divorce rate, and it provides more accurate estimation and tighter confidence
intervals than existing methods
The upper limit of the e+e- partial width of X(3872)
The e+e- decay partial width of the recently observed state, X(3872), is
estimated using the ISR data collected at the center of mass energy 4.03 GeV in
e+e- annihilation experiment by BES at BEPC. It is found that the product of
the e+e- partial width and X(3872) --> pi+ pi- J/psi decay branching fraction
is less than 10 eV at 90 % confidence level if the J(PC) of X(3872) is 1(--).
Together with the potential models and other information, we conclude that
X(3872) is very unlikely to be a vector state.Comment: 5 pages, 1 figur
Recent BES Results on Hadron Spectroscopy
We present recent results from the BES experiment on the observation of the
Y(2175) in J/\psi\to \phi f_0(980) \eta, study of \eta(2225) in J/\psi\to
\gamma \phi \phi, and the production of X(1440) recoiling against an \omega or
a \phi in J/\psi hadronic decays. The observation of \psi(2S) radiative decays
is also presented.Comment: 5 page
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