Approximating Probability Densities by Iterated Laplace Approximations

The Laplace approximation is an old, but frequently used method to approximate integrals for Bayesian calculations. In this paper we develop an extension of the Laplace approximation, by applying it iteratively to the residual, i.e., the difference between the current approximation and the true function. The final approximation is thus a linear combination of multivariate normal densities, where the coefficients are chosen to achieve a good fit to the target distribution. We illustrate on real and artificial examples that the proposed procedure is a computationally efficient alternative to current approaches for approximation of multivariate probability densities. The R-package iterLap implementing the methods described in this article is available from the CRAN servers.Comment: to appear in Journal of Computational and Graphical Statistics, http://pubs.amstat.org/loi/jcg

Photon lifetime in a cavity containing a slow-light medium

We investigate experimentally the lifetime of the photons in a cavity containing a medium exhibiting strong positive dispersion. This intracavity positive dispersion is provided by a metastable helium gas at room temperature in the electromagnetically induced transparency (EIT) regime, in which light propagates at a group velocity of the order of 10000 m/s. The results definitely prove that the lifetime of the cavity photons is governed by the group velocity of light in the cavity, and not its phase velocity.Comment: Accepted for publication in Optics Letter

Acetylene terminated matrix resins

The synthesis of resins with terminal acetylene groups has provided a promising technology to yield high performance structural materials. Because these resins cure through an addition reaction, no volatile by-products are produced during the processing. The cured products have high thermal stability and good properties retention after exposure to humidity. Resins with a wide variety of different chemical structures between the terminal acetylene groups are synthesized and their mechanical properties studied. The ability of the acetylene cured polymers to give good mechanical properties is demonstrated by the resins with quinoxaline structures. Processibility of these resins can be manipulated by varying the chain length between the acetylene groups or by blending in different amounts of reactive deluents. Processing conditions similar to the state-of-the-art epoxy can be attained by using backbone structures like ether-sulfone or bis-phenol-A. The wide range of mechanical properties and processing conditions attainable by this class of resins should allow them to be used in a wide variety of applications

Interacting double dark resonances in a hot atomic vapor of helium

We experimentally and theoretically study two different tripod configurations using metastable helium ($^4$He*), with the probe field polarization perpendicular and parallel to the quantization axis, defined by an applied weak magnetic field. In the first case, the two dark resonances interact incoherently and merge together into a single EIT peak with increasing coupling power. In the second case, we observe destructive interference between the two dark resonances inducing an extra absorption peak at the line center.Comment: 7 pages, 7 figure

Multichip imager with improved optical performance near the butt region

A compound imager consists of two or more individual chips, each with at least one line array of sensors thereupon. Each chip has a glass support plate attached to the side from which light reaches the line arrays. The chips are butted together end-to-end to make large line arrays of sensors. Because of imperfections in cutting, the butted surfaces define a gap. Light entering in the region of the gap is either lost or falls on an individual imager other than the one for which it is intended. This results in vignetting and/or crosstalk near the butted region. The gap is filled with an epoxy resin or other similar material which, when hardened, has an index of referaction near that of the glass support plate

How does quality enter into health care purchasing decisions?

A number of employers, business consortia, and public purchasers are promoting value-based purchasing as a way to improve the quality of patient care. Some purchasers are using publicly available information on health plan and provider performance to make their health plan and provider choices, while others are using their market power to drive improvements in patient care and safety. This article examines six key strategies used by purchasers

Ferric Gluconate Yields Cost-Savings in Hemodialysis Patients with High Ferritin and Low TSAT: Results from the DRIVE Studies

Purpose: One third of hemodialysis patients have high serum ferritin levels and low transferrin saturation (TSAT). The purpose of this analysis was to determine the cost effectiveness of administering 1g of sodium ferric gluconate complex (SFGC: also referred to as ferric gluconate) to patients with serum ferritin \u3e500ng/mL and TSAT ≤25% based on the Dialysis Patients Response to IV Iron with Elevated Ferritin (DRIVE) study and its 6-week observational extension (DRIVE-II). In these studies, IV iron administration resulted in reduced epoetin requirements. Methods: Decision analysis was performed using a time horizon of 12 weeks, consistent with the combined duration of DRIVE and DRIVE II. Treatment effectiveness was based on mean increase in hemoglobin (Hb) for each group (SFGC plus epoetin or epoetin alone) in the intention to treat populations. Costs included drugs (SFGC and epoetin) and hospitalizations due to serious adverse events (SAEs) reported. The primary cost effectiveness measure was cost per g/dL of Hb increase at 12 weeks. Costs were computed from a Medicare perspective using projected 2007 reimbursements. Sensitivity analyses were performed to test the impact of using the safety population, median epoetin and SFGC doses, actual 2005 Medicare reimbursements, median increases in Hb, and SAE rate changes. The model was constructed using TreeAge Pro software. Results: Total cost per patient receiving SFGC plus epoetin was $3675 per g/dL Hb increase, while the total cost per patient receiving epoetin alone was$5065 per g/dL Hb increase. Net savings for SFGC plus epoetin was $1390 per g/dL Hb increase over the 12 week period Sensitivity analyses affirmed the robustness of the model. Conclusion: Administering 1g of SFGC plus epoetin in patients with high ferritin and low TSAT as defined in the DRIVE studies resulted in significant cost-savings compared to epoetin alone