Maximum likelihood and pseudo score approaches for parametric time-to-event analysis with informative entry times

We develop a maximum likelihood estimating approach for time-to-event Weibull regression models with outcome-dependent sampling, where sampling of subjects is dependent on the residual fraction of the time left to developing the event of interest. Additionally, we propose a two-stage approach which proceeds by iteratively estimating, through a pseudo score, the Weibull parameters of interest (i.e., the regression parameters) conditional on the inverse probability of sampling weights; and then re-estimating these weights (given the updated Weibull parameter estimates) through the profiled full likelihood. With these two new methods, both the estimated sampling mechanism parameters and the Weibull parameters are consistently estimated under correct specification of the conditional referral distribution. Standard errors for the regression parameters are obtained directly from inverting the observed information matrix in the full likelihood specification and by either calculating bootstrap or robust standard errors for the hybrid pseudo score/profiled likelihood approach. Loss of efficiency with the latter approach is considered. Robustness of the proposed methods to misspecification of the referral mechanism and the time-to-event distribution is also briefly examined. Further, we show how to extend our methods to the family of parametric time-to-event distributions characterized by the generalized gamma distribution. The motivation for these two approaches came from data on time to cirrhosis from hepatitis C viral infection in patients referred to the Edinburgh liver clinic. We analyze these data here.Comment: Published in at http://dx.doi.org/10.1214/14-AOAS725 the Annals of Applied Statistics (http://www.imstat.org/aoas/) by the Institute of Mathematical Statistics (http://www.imstat.org

Phenomenon of Alfvénic vortex shedding

Generation of Alfvenic (magnetohydrodynamic) vortices by the interaction of compressible plasma flows with magnetic-field-aligned blunt obstacles is modeled in terms of magnetohydrodynamics. It is found that periodic shedding of vortices with opposite vorticity is a robust feature of the interaction in a broad range of plasma parameters: for plasma beta from 0.025 to 0.5, and for the flow speeds from 0.1 to 0.99 of the fast magnetoacoustic speed. The Strouhal number is the dimensionless ratio of the blunt body diameter to the product of the period of vortex shedding and the inflow speed. It is found to be consistently in the range 0.15-0.25 in the whole range of parameters. The induced Alfvenic vortices are compressible and contain spiral-armed perturbations of the magnetic field strength and plasma mass density up to 50%-60% of the background values. The generated electric current also has the spiral-armed structuring

How does a quadratic term in the energy dispersion modify the single-particle Green's function of the Tomonaga-Luttinger model?

We calculate the effect of a quadratic term in the energy dispersion on the low-energy behavior of the Green's function of the spinless Tomonaga-Luttinger model (TLM). Assuming that for small wave-vectors q = k - k_F the fermionic excitation energy relative to the Fermi energy is v_F q + q^2 / (2m), we explicitly calculate the single-particle Green's function for finite but small values of lambda = q_c /(2k_F). Here k_F is the Fermi wave-vector, q_c is the maximal momentum transfered by the interaction, and v_F = k_F / m is the Fermi velocity. Assuming equal forward scattering couplings g_2 = g_4, we find that the dominant effect of the quadratic term in the energy dispersion is a renormalization of the anomalous dimension. In particular, at weak coupling the anomalous dimension is tilde{gamma} = gamma (1 - 2 lambda^2 gamma), where gamma is the anomalous dimension of the TLM. We also show how to treat the change of the chemical potential due to the interactions within the functional bosonization approach in arbitrary dimensions.Comment: 17 pages, 1 figur

The Hystery Unit - A Short Term Memory Model for Computational Neurons

In this paper, a model of short term memory is introduced. This model is inspired by the transient behavior of neurons and magnetic storage as memory. The transient response of a neuron is hypothesized to be a combination of a pair of sigmoids, and a relation is drawn to the hysteresis loop characteristics of magnetic materials. A model is created as a composition of two coupled families of curves. Two theorems are derived regarding the asymptotic convergence behavior of the model. Another conjecture claims that the model retains full memory of all past unit step inputs

Robust Charge-based Qubit Encoding

We propose a simple encoding of charge-based quantum dot qubits which protects against fluctuating electric fields by charge symmetry. We analyse the reduction of coupling to noise due to nearby charge traps and present single qubit gates. The relative advantage of the encoding increases with lower charge trap density.Comment: 6 Pages, 7 Figures. Published Versio

High efficiency InGaAs solar cells on Si by InP layer transfer

InP/Si substrates were fabricated through wafer bonding and helium-induced exfoliation of InP, and InGaAs solar cells lattice matched to bulk InP were grown on these substrates using metal-organic chemical-vapor deposition. The photovoltaic characteristics of the InGaAs cells fabricated on the wafer-bonded InP/Si substrates were comparable to those synthesized on commercially available epiready InP substrates, thus providing a demonstration of wafer-bonded InP/Si substrates as an alternative to bulk InP substrates for solar cell applications

Characterization of frequency drift of sampled-grating DBR laser module under direct modulation

The authors demonstrate the drift in frequency of a static sampled-grating distributed Bragg reflector (SG DBR) laser module when it is subjected to direct modulation. The magnitude of drift and its settling time is characterized as a function of the index of modulation. Results show that when the directly modulated SG DBR is optically filtered, as in a dense wavelength- division- multiplexed system, a power penalty of 6.7 dB is incurred in comparison to the unfiltered case

Use of Programmed Piezo Crystal Flexures for Economic Vapor Deposition of Parylene HT® on Unlimited Lengths of Magnet Wire

The electronics industry recognizes the need for high-temperature electronics (HTE) particularly for aerospace and geothermal applications. HTE is generally defined as robust operation in temperatures up to 300°C. A major constraint to HTE is high temperature magnet wire which is pervasive in electronic component windings and signal wire for sensors. The magnet wire constraint is caused by the temperature limits of the thin Polytetrafluoroethylene (PTFE) and Fluorinated Ethylene Propylene (FEP) coatings applied to HT magnet wire that limits the operating temperature to 220°C. [1], [2] There are coatings, particularly parylene-based coatings such as parylene HT®, that would greatly improve HT magnet wire, signal wire, and create the potential for subminiature thermocouple (TC) sensors; however, the slow vapor deposition process required to apply parylene is generally thought impractical for use in pore-free coating of long lengths of small diameter wire. For this research, experiments were first performed coating small diameter, wire product prototypes in standard batch vacuum chambers utilizing static fixtures. Finding this approach impractical we devised a new process utilizing a piezo-crystal electrodynamically actuated fixture of 14” diameter by 18” height that supports a web of one 24,500’ long, continuous small-diameter wire. A prototype dynamic fixture was built and a trial run successfully coated a 1500’ length of 0.005” diameter copper wire with Parylene HT®. This successful demonstration was the basis for a DOL Phase I SBIR to explore the feasibility of electrodynamically actuated devises that would synchronize horizontal and vertical actuation to drive horizontal motion to the wire web to enable a continuous reel-to-reel operation for parylene vapor deposition. This is discussed in future work