8,545 research outputs found
Building Prediction Models for Dementia: The Need to Account for Interval Censoring and the Competing Risk of Death
Indiana University-Purdue University Indianapolis (IUPUI)Context. Prediction models for dementia are crucial for informing clinical decision making in older adults. Previous models have used genotype and age to obtain risk scores to determine risk of Alzheimer’s Disease, one of the most common forms of dementia (Desikan et al., 2017). However, previous prediction models do not account for the fact that the time to dementia onset is unknown, lying between the last negative and the first positive dementia diagnosis time (interval censoring). Instead, these models use time to diagnosis, which is greater than or equal to the true dementia onset time. Furthermore, these models do not account for the competing risk of death which is quite frequent among elder adults.
Objectives. To develop a prediction model for dementia that accounts for interval censoring and the competing risk of death. To compare the predictions from this model with the predictions from a naïve analysis that ignores interval censoring and the competing risk of death.
Methods. We apply the semiparametric sieve maximum likelihood (SML) approach to simultaneously model the cumulative incidence function (CIF) of dementia and death while accounting for interval censoring (Bakoyannis, Yu, & Yiannoutsos, 2017). The SML is implemented using the R package intccr. The CIF curves of dementia are compared for the SML and the naïve approach using a dataset from the Indianapolis Ibadan Dementia Project.
Results. The CIF from the SML and the naïve approach illustrated that for healthier individuals at baseline, the naïve approach underestimated the incidence of dementia compared to the SML, as a result of interval censoring. Individuals with a poorer health condition at baseline have a CIF that appears to be overestimated in the naïve approach. This is due to older individuals with poor health conditions having an elevated risk of death.
Conclusions. The SML method that accounts for the competing risk of death along with interval censoring should be used for fitting prediction/prognostic models of dementia to inform clinical decision making in older adults. Without controlling for the competing risk of death and interval censoring, the current models can provide invalid predictions of the CIF of dementia
Finding bright <i>z</i> ≥ 6.6 Ly <i>α</i> emitters with lensing: prospects for <i>Euclid</i>
We model the Ly luminosity function to estimate the
number of lensed high Ly emitters that may be detected by the
Euclid Deep Survey. To span the whole range of possible predictions we exploit
two Ly luminosity function models and two strong gravitational lensing
models from the literature. We show that the planned Euclid Deep Survey
observing 40 deg over the 920-1850 nm wavelength range down to a flux limit
of erg scm will enable us to find
between and deg lensed Ly emitters at depending on the adopted Ly luminosity function and strong
gravitational lensing model. The obvious [OII], [OIII] and H
contaminants of the Ly lensed population will be identified with the
help of Euclid's spectral resolving power, while the SKA will enable the
identification of the interloper population of H emitters. By combining
Euclid and the SKA, we will thus be able to identify, for the first time, a
sample of to lensed Ly emitters at .Comment: Accepted for publication in MNRAS on 20 June 2017. (NEW: Amended
Latex
Non-BCS superconductivity for underdoped cuprates by spin-vortex attraction
Within a gauge approach to the t-J model, we propose a new, non-BCS mechanism
of superconductivity for underdoped cuprates. The gluing force of the
superconducting mechanism is an attraction between spin vortices on two
different N\'eel sublattices, centered around the empty sites described in
terms of fermionic holons. The spin fluctuations are described by bosonic
spinons with a gap generated by the spin vortices. Due to the no-double
occupation constraint, there is a gauge attraction between holon and spinon
binding them into a physical hole. Through gauge interaction the spin vortex
attraction induces the formation of spin-singlet (RVB) spin pairs with a
owering of the spinon gap. Lowering the temperature the approach exhibits two
crossover temperatures: at the higher crossover a finite density of incoherent
holon pairs are formed leading to a reduction of the hole spectral weight, at
the lower crossover also a finite density of incoherent spinon RVB pairs are
formed, giving rise to a gas of incoherent preformed hole pairs, and magnetic
vortices appear in the plasma phase. Finally, at a even lower temperature the
hole pairs become coherent, the magnetic vortices become dilute and
superconductivity appears. The superconducting mechanism is not of BCS-type
since it involves a gain in kinetic energy (for spinons) coming from the spin
interactions.Comment: 4 pages, 3 figures, accepted by the proceedings of SNS2010 conferenc
Superfluidity, Sound Velocity and Quasi Condensation in the 2D BCS-BEC Crossover
We study finite-temperature properties of a two-dimensional superfluid made
of ultracold alkali-metal atoms in the BCS-BEC crossover. We investigate the
region below the critical temperature of the
Berezinskii-Kosterlitz-Thouless phase transition, where there is
quasi-condensation, by analyzing the effects of phase and amplitude
fluctuations of the order parameter. In particular, we calculate the superfluid
fraction, the sound velocity and the quasi-condensate fraction as a function of
the temperature and of the binding energy of fermionic pairs.Comment: 7 pages, 4 figures, improved version to be published in Phys. Rev.
Spin-charge gauge symmetry: A way to tackle HTS cuprates?
We propose an explanation of several experimental features of transport
phenomena in the normal state of high Tc cuprates in terms of a spin-charge
gauge theory of the 2D t-J model. The calculated doping-temperature dependence
for a number of physical quantities is found in qualitative agreement with
data. In particular, we recover: in the ``pseudogap phase'' the metal-insulator
crossover of the in-plane resistivity and of the NMR ``relaxation time'' and
the insulating behavior of the out-of-plane resistivity; in the ``strange metal
phase'' (at higher temperature or doping) the linear in T behavior of the above
quantities; the appearance of maxima in the in-plane far-infrared conductivity
in strongly underdoped and overdoped samples.Comment: 8 pages, 14 figures; to appear without figures in Journal of Physics
and Chemistry of Solid
Beliaev damping of the Goldstone mode in atomic Fermi superfluids
Beliaev damping in a superfluid is the decay of a collective excitation into
two lower frequency collective excitations; it represents the only decay mode
for a bosonic collective excitation in a superfluid at T = 0. The standard
treatment for this decay assumes a linear spectrum, which in turn implies that
the final state momenta must be collinear to the initial state. We extend this
treatment, showing that the inclusion of a gradient term in the Hamiltonian
yields a realistic spectrum for the bosonic excitations; we then derive a
formula for the decay rate of such excitations, and show that even moderate
nonlinearities in the spectrum can yield substantial deviations from the
standard result. We apply our result to an attractive Fermi gas in the BCS-BEC
crossover: here the low-energy bosonic collective excitations are density
oscillations driven by the phase of the pairing order field. These collective
excitations, which are gapless modes as a consequence of the Goldstone
mechanism, have a spectrum which is well established both theoretically and
experimentally, and whose linewidth, we show, is determined at low temperatures
by the Beliaev decay mechanism.Comment: 8 pages, 3 figure
Transport Properties in the "Strange Metal Phase" of High Tc Cuprates: Spin-Charge Gauge Theory Versus Experiments
The SU(2)xU(1) Chern-Simons spin-charge gauge approach developed earlier to
describe the transport properties of the cuprate superconductors in the
``pseudogap'' regime, in particular, the metal-insulator crossover of the
in-plane resistivity, is generalized to the ``strange metal'' phase at higher
temperature/doping. The short-range antiferromagnetic order and the gauge field
fluctuations, which were the key ingredients in the theory for the pseudogap
phase, also play an important role in the present case. The main difference
between these two phases is caused by the existence of an underlying
statistical -flux lattice for charge carriers in the former case, whereas
the background flux is absent in the latter case. The Fermi surface then
changes from small ``arcs'' in the pseudogap to a rather large closed line in
the strange metal phase. As a consequence the celebrated linear in T dependence
of the in-plane and out-of-plane resistivity is shown explicitly to recover.
The doping concentration and temperature dependence of theoretically calculated
in-plane and out-of-plane resistivity, spin-relaxation rate and AC conductivity
are compared with experimental data, showing good agreement.Comment: 14 pages, 5 .eps figures, submitted to Phys. Rev. B, revised version
submitted on 24 Oc
Contractile stresses in cohesive cell layers on finite-thickness substrates
Using a minimal model of cells or cohesive cell layers as continuum active
elastic media, we examine the effect of substrate thickness and stiffness on
traction forces exerted by strongly adhering cells. We obtain a simple
expression for the length scale controlling the spatial variation of stresses
in terms of cell and substrate parameters that describes the crossover between
the thin and thick substrate limits. Our model is an important step towards a
unified theoretical description of the dependence of traction forces on cell or
colony size, acto-myosin contractility, substrate depth and stiffness, and
strength of focal adhesions, and makes experimentally testable predictions.Comment: 5 pages, 3 figure
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