278 research outputs found
Estimating medical costs from a transition model
Nonparametric estimators of the mean total cost have been proposed in a
variety of settings. In clinical trials it is generally impractical to follow
up patients until all have responded, and therefore censoring of patient
outcomes and total cost will occur in practice. We describe a general
longitudinal framework in which costs emanate from two streams, during sojourn
in health states and in transition from one health state to another. We
consider estimation of net present value for expenditures incurred over a
finite time horizon from medical cost data that might be incompletely
ascertained in some patients. Because patient specific demographic and clinical
characteristics would influence total cost, we use a regression model to
incorporate covariates. We discuss similarities and differences between our net
present value estimator and other widely used estimators of total medical
costs. Our model can accommodate heteroscedasticity, skewness and censoring in
cost data and provides a flexible approach to analyses of health care cost.Comment: Published in at http://dx.doi.org/10.1214/193940307000000266 the IMS
Collections (http://www.imstat.org/publications/imscollections.htm) by the
Institute of Mathematical Statistics (http://www.imstat.org
A superconducting microwave multivibrator produced by coherent feedback
We investigate a coherent nonlinear feedback circuit constructed from
pre-existing superconducting microwave devices. The network exhibits emergent
bistable and astable states, and we demonstrate its operation as a latch and
the frequency locking of its oscillations. While the network is tedious to
model by hand, our observations agree quite well with the semiclassical
dynamical model produced by a new software package [N. Tezak et al.,
arXiv:1111.3081v1] that systematically interpreted an idealized schematic of
the system as a quantum optic feedback network.Comment: 9 double-spaced pages, 5 figures and supplement. To appear in Phys.
Rev. Let
Efficient multiple time scale molecular dynamics: using colored noise thermostats to stabilize resonances
Multiple time scale molecular dynamics enhances computational efficiency by
updating slow motions less frequently than fast motions. However, in practice
the largest outer time step possible is limited not by the physical forces but
by resonances between the fast and slow modes. In this paper we show that this
problem can be alleviated by using a simple colored noise thermostatting scheme
which selectively targets the high frequency modes in the system. For two
sample problems, flexible water and solvated alanine dipeptide, we demonstrate
that this allows the use of large outer time steps while still obtaining
accurate sampling and minimizing the perturbation of the dynamics. Furthermore,
this approach is shown to be comparable to constraining fast motions, thus
providing an alternative to molecular dynamics with constraints.Comment: accepted for publication by the Journal of Chemical Physic
Magnet field sensing beyond the standard quantum limit under the effect of decoherence
Entangled states can potentially be used to outperform the standard quantum
limit which every classical sensor is bounded by. However, entangled states are
very susceptible to decoherence, and so it is not clear whether one can really
create a superior sensor to classical technology via a quantum strategy which
is subject to the effect of realistic noise. This paper presents an
investigation of how a quantum sensor composed of many spins is affected by
independent dephasing. We adopt general noise models including non-Markovian
effects, and in these noise models the performance of the sensor depends
crucially on the exposure time of the sensor to the field. We have found that,
by choosing an appropriate exposure time within non-Markovian time region, an
entangled sensor does actually beat the standard quantum limit. Since
independent dephasing is one of the most typical sources of noise in many
systems, our results suggest a practical and scalable approach to beating the
standard quantum limit
Intrinsic noise and discrete-time processes
A general formalism is developed to construct a Markov chain model that
converges to a one-dimensional map in the infinite population limit. Stochastic
fluctuations are therefore internal to the system and not externally specified.
For finite populations an approximate Gaussian scheme is devised to describe
the stochastic fluctuations in the non-chaotic regime. More generally, the
stochastic dynamics can be captured using a stochastic difference equation,
derived through an approximation to the Markov chain. The scheme is
demonstrated using the logistic map as a case study.Comment: Modified version accepted for publication in Phys. Rev. E Rapid
Communications. New figures adde
Efficient growth of complex graph states via imperfect path erasure
Given a suitably large and well connected (complex) graph state, any quantum
algorithm can be implemented purely through local measurements on the
individual qubits. Measurements can also be used to create the graph state:
Path erasure techniques allow one to entangle multiple qubits by determining
only global properties of the qubits. Here, this powerful approach is extended
by demonstrating that even imperfect path erasure can produce the required
graph states with high efficiency. By characterizing the degree of error in
each path erasure attempt, one can subsume the resulting imperfect entanglement
into an extended graph state formalism. The subsequent growth of the improper
graph state can be guided, through a series of strategic decisions, in such a
way as to bound the growth of the error and eventually yield a high-fidelity
graph state. As an implementation of these techniques, we develop an analytic
model for atom (or atom-like) qubits in mismatched cavities, under the
double-heralding entanglement procedure of Barrett and Kok [Phys. Rev. A 71,
060310 (2005)]. Compared to straightforward postselection techniques our
protocol offers a dramatic improvement in growing complex high-fidelity graph
states.Comment: 15 pages, 10 figures (which print to better quality than when viewed
as an on screen pdf
A Study of the Persistence of Mycobacterium bovis in the Environment under Natural Weather Conditions in Michigan, USA
Reisolation of Mycobacterium bovis from inoculated substrates was used to follow the persistence of viable M. bovis bacteria exposed to natural weather conditions over a 12-month period. Environmental factors were recorded continuously, and factors affecting M. bovis persistence (i.e., temperature, season, and substrate) were studied using survival analysis and Cox's proportional hazards regression. Persistence of M. bovis in the environment was significantly shorter in the spring/summer season, characterized by the highest average daily temperatures over the 12-month period. M. bovis persisted up to 88 days in soil, 58 days in water and hay, and 43 days on corn. These studies demonstrate that M. bovis bacteria persist long enough to represent a risk of exposure for cattle and/or wildlife and strengthen evidence that suggests cattle farm biosecurity and efforts to eliminate supplemental feeding of white-tailed deer will decrease the risk of bovine TB transmission among and between cattle and deer populations
Longitudinal analysis of censored medical cost data
This paper applies the inverse probability weighted (IPW) least-squares method to estimate the effects of treatment on total medical cost, subject to censoring, in a panel-data setting. IPW pooled ordinary-least squares (POLS) and IPW random effects (RE) models are used. Because total medical cost might not be independent of survival time under administrative censoring, unweighted POLS and RE cannot be used with censored data, to assess the effects of certain explanatory variables. Even under the violation of this independency, IPW estimation gives consistent asymptotic normal coefficients with easily computable standard errors. A traditional and robust form of the Hausman test can be used to compare weighted and unweighted least squares estimators. The methods are applied to a sample of 201 Medicare beneficiaries diagnosed with lung cancer between 1994 and 1997
Thermal Flipping of Interstellar Grains
In interstellar dust grains, internal processes dissipate rotational kinetic
energy. The dissipation is accompanied by thermal fluctuations, which transfer
energy from the vibrational modes to rotation. Together, these processes are
known as internal relaxation. For the past several years, internal relaxation
has been thought to give rise to thermal flipping, with profound consequences
for grain alignment theory. I show that thermal flipping is not possible in the
limit that the inertia tensor does not vary with time.Comment: 5 pages, accepted by Ap
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