1,114 research outputs found

    Bayesian modeling longitudinal dyadic data with nonignorable dropout, with application to a breast cancer study

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    Dyadic data are common in the social and behavioral sciences, in which members of dyads are correlated due to the interdependence structure within dyads. The analysis of longitudinal dyadic data becomes complex when nonignorable dropouts occur. We propose a fully Bayesian selection-model-based approach to analyze longitudinal dyadic data with nonignorable dropouts. We model repeated measures on subjects by a transition model and account for within-dyad correlations by random effects. In the model, we allow subject's outcome to depend on his/her own characteristics and measure history, as well as those of the other member in the dyad. We further account for the nonignorable missing data mechanism using a selection model in which the probability of dropout depends on the missing outcome. We propose a Gibbs sampler algorithm to fit the model. Simulation studies show that the proposed method effectively addresses the problem of nonignorable dropouts. We illustrate our methodology using a longitudinal breast cancer study.Comment: Published in at http://dx.doi.org/10.1214/11-AOAS515 the Annals of Applied Statistics (http://www.imstat.org/aoas/) by the Institute of Mathematical Statistics (http://www.imstat.org

    Investigation of the potential catalytic activities of several metal organic framework (MOF) materials

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    Metal-organic frameworks (MOFs) are promising porous materials due to their high porosities, large apparent surface areas, tunable pore properties and selective uptake of small molecules. Many researches have been focused on developing new MOF structures and exploring their use in gas storage and separation. Only few studies investigated the catalytic properties of MOFs. In this study, catalytic properties are explored over several MOFs including zeolitic imidazolate framework-8 (ZIF-8), Ni(FA) and Zn(FA). These MOFs are used as catalyst supports for iron or platinum nanoparticles. The catalysts are prepared using incipient wetness impregnation method and characterized with Brunauer-Emmett-Teller (BET), pulse chemisorption, X-ray diffraction (XRD), temperature-programmed reduction (TPR), and temperature-programmed desorption (TPD). The catalysts are tested from the water gas shift and methanol oxidation reactions using a fixed bed flowing reactor at 1 atm. The characterization results show that these MOFs decompose at relatively low temperatures. These materials did not have good activities for the water gas shift reaction under the operation conditions. Based on the methanol TPD results, Pt-ZIF8 showed some methanol oxidation activity. The low activity is possibly due to the low operation temperatures since the operation temperatures are limited by the stability of the MOF materials. This pioneering work indicates that it is critical to improve the stability of the MOFs during catalyst preparation and under the reaction conditions for their application in catalysis field

    On Nonlinear Control Perspectives of a Challenging Benchmark

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    From Type-II Triply Degenerate Nodal Points and Three-Band Nodal Rings to Type-II Dirac Points in Centrosymmetric Zirconium Oxide

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    Using first-principles calculations, we report that ZrO is a topological material with the coexistence of three pairs of type-II triply degenerate nodal points (TNPs) and three nodal rings (NRs), when spin-orbit coupling (SOC) is ignored. Noticeably, the TNPs reside around Fermi energy with large linear energy range along tilt direction (> 1 eV) and the NRs are formed by three strongly entangled bands. Under symmetry-preserving strain, each NR would evolve into four droplet-shaped NRs before fading away, producing distinct evolution compared with that in usual two-band NR. When SOC is included, TNPs would transform into type-II Dirac points while all the NRs have gaped. Remarkably, the type-II Dirac points inherit the advantages of TNPs: residing around Fermi energy and exhibiting large linear energy range. Both features facilitate the observation of interesting phenomena induced by type-II dispersion. The symmetry protections and low-energy Hamiltonian for the nontrivial band crossings are discussed.Comment: 7 pages, 5 figures, J. Phys. Chem. Lett. 201
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