7,665 research outputs found
The modeling of medical expenditure data from a longitudinal survey using the generalized method of moments (GMM) approach
Medical expenditure data analysis has recently become an important problem in biostatistics. These data typically have a number of features making their analysis rather difficult. Commonly, they are heavily right-skewed, contain a large percentage of zeros, and often exhibit large numbers of missing observations because of death and/or the lack of follow-up. They are also commonly obtained from records that are linked to large longitudinal data surveys. In this manuscript, we suggest a novel approach to modeling these data through the use of generalized method of moments estimation procedure combined with appropriate weights that account for both dropout due to death and the probability of being sampled from among the National Long Term Care Survey (NLTCS) subjects. This approach seems particularly appropriate because of the large number of subjects relative to the length of observation period (in months). We also use a simulation study to compare our proposed approach with and without the use of weights. The proposed model is applied to medical expenditure data obtained from the 2004-2005 NLTCS-linked Medicare database. The results suggest that the amount of medical expenditures incurred is strongly associated with higher number of activities of daily living (ADL) disabilities and self-reports of unmet need for help with ADL disabilities
Density-Polarization Functional Theory of the response of a periodic insulating solid to an electric field.
The response of an infinite, periodic, insulating, solid to an
infinitesimally small electric field is investigated in the framework of
Density Functional Theory. We find that the applied perturbing potential is not
a unique functional of the periodic density change~: it depends also on the
change in the macroscopic {\em polarization}. Moreover, the dependence of the
exchange-correlation energy on polarization induces an exchange-correlation
electric field. These effects are exhibited for a model semiconductor. We also
show that the scissor-operator technique is an approximate way of bypassing
this polarization dependence.Comment: 11 pages, 1 Fig
Second harmonic generation in SiC polytypes
LMTO calculations are presented for the frequency dependent second harmonic
generation (SHG) in the polytypes 2H, 4H, 6H, 15R and 3C of SiC. All
independent tensor components are calculated. The spectral features and the
ratios of the 333 to 311 tensorial components are studied as a function of the
degree of hexagonality. The relationship to the linear optical response and the
underlying band structure are investigated. SHG is suggested to be a sensitive
tool for investigating the near band edge interband excitations.Comment: 12 pages, 10 figure
Operation of a quantum dot in the finite-state machine mode: single-electron dynamic memory
A single electron dynamic memory is designed based on the non-equilibrium
dynamics of charge states in electrostatically-defined metallic quantum dots.
Using the orthodox theory for computing the transfer rates and a master
equation, we model the dynamical response of devices consisting of a charge
sensor coupled to either a single and or a double quantum dot subjected to a
pulsed gate voltage. We show that transition rates between charge states in
metallic quantum dots are characterized by an asymmetry that can be controlled
by the gate voltage. This effect is more pronounced when the switching between
charge states corresponds to a Markovian process involving electron transport
through a chain of several quantum dots. By simulating the dynamics of electron
transport we demonstrate that the quantum box operates as a finite-state
machine that can be addressed by choosing suitable shapes and switching rates
of the gate pulses. We further show that writing times in the ns range and
retention memory times six orders of magnitude longer, in the ms range, can be
achieved on the double quantum dot system using experimentally feasible
parameters thereby demonstrating that the device can operate as a dynamic
single electron memory.Comment: 18 pages, 8 figure
Zero-range process with open boundaries
We calculate the exact stationary distribution of the one-dimensional
zero-range process with open boundaries for arbitrary bulk and boundary hopping
rates. When such a distribution exists, the steady state has no correlations
between sites and is uniquely characterized by a space-dependent fugacity which
is a function of the boundary rates and the hopping asymmetry. For strong
boundary drive the system has no stationary distribution. In systems which on a
ring geometry allow for a condensation transition, a condensate develops at one
or both boundary sites. On all other sites the particle distribution approaches
a product measure with the finite critical density \rho_c. In systems which do
not support condensation on a ring, strong boundary drive leads to a condensate
at the boundary. However, in this case the local particle density in the
interior exhibits a complex algebraic growth in time. We calculate the bulk and
boundary growth exponents as a function of the system parameters
The TWA 3 Young Triple System: Orbits, Disks, Evolution
We have characterized the spectroscopic orbit of the TWA 3A binary and
provide preliminary families of probable solutions for the TWA 3A visual orbit
as well as for the wide TWA 3A--B orbit. TWA 3 is a hierarchical triple located
at 34 pc in the 10 Myr old TW Hya association. The wide component
separation is 1."55; the close pair was first identified as a possible binary
almost 20 years ago. We initially identified the 35-day period orbital solution
using high-resolution infrared spectroscopy which angularly resolved the A and
B components. We then refined the preliminary orbit by combining the infrared
data with a re-analysis of our high-resolution optical spectroscopy. The
orbital period from the combined spectroscopic solution is 35 days, the
eccentricity is 0.63, and the mass ratio is 0.84; although this
high mass ratio would suggest that optical spectroscopy alone should be
sufficient to identify the orbital solution, the presence of the tertiary B
component likely introduced confusion in the blended optical spectra. Using
millimeter imaging from the literature, we also estimate the inclinations of
the stellar orbital planes with respect to the TWA 3A circumbinary disk
inclination and find that all three planes are likely misaligned by at least
30 degrees. The TWA 3A spectroscopic binary components have spectral
types of M4.0 and M4.5; TWA 3B is an M3. We speculate that the system formed as
a triple, is bound, and that its properties were shaped by dynamical
interactions between the inclined orbits and disk.Comment: Accepted to Ap
Power Law Distribution of Wealth in a Money-Based Model
A money-based model for the power law distribution (PLD) of wealth in an
economically interacting population is introduced. The basic feature of our
model is concentrating on the capital movements and avoiding the complexity of
micro behaviors of individuals. It is proposed as an extension of the Equiluz
and Zimmermann's (EZ) model for crowding and information transmission in
financial markets. Still, we must emphasize that in EZ model the PLD without
exponential correction is obtained only for a particular parameter, while our
pattern will give it within a wide range. The Zipf exponent depends on the
parameters in a nontrivial way and is exactly calculated in this paper.Comment: 5 pages and 4 figure
Exact Kohn-Sham exchange kernel for insulators and its long-wavelength behavior
We present an exact expression for the frequency-dependent Kohn-Sham
exact-exchange (EXX) kernel for periodic insulators, which can be employed for
the calculation of electronic response properties within time-dependent (TD)
density-functional theory. It is shown that the EXX kernel has a
long-wavelength divergence behavior of the exact full exchange-correlation
kernel and thus rectifies one serious shortcoming of the adiabatic
local-density approximation and generalized-gradient approximations kernels. A
comparison between the TDEXX and the GW-approximation-Bethe-Salpeter-equation
approach is also made.Comment: two column format 6 pages + 1 figure, to be publisehd in Physical
Review
Born Effective Charges and Infrared Response of LiBC
Calculations of the zone center optical mode frequencies (including LO-TO
splitting), Born effective charges Z for each atom,
dielectric constants and , and the dielectric
response in the infrared, using density functional linear response theory, are
reported. Calculated Raman modes are in excellent agreement with experimental
values (170 cm and 1170 cm), while it will require better
experimental data to clarify the infrared active mode frequencies. The Born
effective charges Z (i) have surprisingly different values
for B and C, and (ii) show considerable anisotropy. Relationships between the
effective charges and LO-TO splitting are discussed, and the predicted
reflectivity in the range 0 -- 1400 cm is presented. These results hold
possible implications for Li removal in LiBC, and C substition for B in
MgB.Comment: 6 pages, 3 figure
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