190,633 research outputs found
A comparison of two methods of estimating propensity scores after multiple imputation
No full textIn many observational studies, analysts estimate treatment effects using propensity scores, e.g. by matching or sub-classifying on the scores. When some values of the covariates are missing, analysts can use multiple imputation to fill in the missing data, estimate propensity scores based on the m completed datasets, and use the propensity scores to estimate treatment effects. We compare two approaches to implement this process. In the first, the analyst estimates the treatment effect using propensity score matching within each completed data set, and averages the m treatment effect estimates. In the second approach, the analyst averages the m propensity scores for each record across the completed datasets, and performs propensity score matching with these averaged scores to estimate the treatment effect. We compare properties of both methods via simulation studies using artificial and real data. The simulations suggest that the second method has greater potential to produce substantial bias reductions than the first, particularly when the missing values are predictive of treatment assignment
The papaya: botany, production and uses/ edited by Sisir Mitra
No full textIncludes bibliographical references and index"Global papaya production has grown significantly over the last few years, mainly as a result of increased production in India. This is the first comprehensive book authored by an international team of experts at the forefront of research and covers botany, biotechnology, production, postharvest physiology and processing"--Origin, History, Composition and Processing / S.K.Mitra, G.Fuentes, A. Chan, and Jorge M.Santamaria -- Production and Trade / Edward A. Evans and Fredy H.Ballan -- Taxonomy, Botany and Plant Development / Víctor M. Jiménez, Marco V. Gutiérrez-Soto, Luis Barboza-Barquero and Eric Guevara -- Propagation / Sisir Mitra -- Biotechnology / Chutchamas Kanchana-udomkan, UsanaNantawan, Rod Drew and Rebecca Ford -- Varieties and crop improvement / Sisir Mitra and S.K.Sharma -- Nutrition and Irrigation / Sisir Mitra -- Plant Water Relations / Amaranta Giron, Christian Alcocer, Gabriela Fuentes, Francisco Espadas, Carlos Talavera, Humberto Estrella, Sisir Mitra and Jorge M Santamaria -- Physiology and Growing Environment on Productivity / Ziwei Zhou, Rebecca Ford and Chutchamas Kanchana-udomkan -- Flowering and Sex expression / Jian-Zhi Huang, Ting-Chi Cheng, Wen-Li Lee, Chen-Yu Lee, Shih Wen Chin, and Fure-Chyi Chen -- Fruit set, development, maturity and ripening / Eder Dutra de Resende, Julián CuevasGonzález, João Paulo Fabi -- Physiological disorders / N.E.Tajidin, M. Munirah, N.I.A. Shukor, and S.H. Ahmad -- Insect and Nematode Pests / Matiyar Rahaman Khan and Amalendu Ghosh -- Diseases / Lynton L. Vawdrey and Paul R. Campbell -- Greenhouse Cultivation / Vìctor Galán Saúco and Eudaldo Pérez Hernández -- Postharvest Handling, Storage and Quality / Pauziah Muda, Nancy Jung Chen and Robert E. Paull1 online resource (xiii, 269 pages
Guava: botany, production and uses Botany, production and uses./ edited by Sisir Mitra.
No full textIncludes bibliographical references and index."Guava is considered a minor tropical fruit (together with lychee, longan, durian etc.), although is the largest in terms of output. This is the first comprehensive book authored by an international team of experts, and covers botany, biotechnology, propagation, production, pests and diseases, postharvest, and processing"--Psidium guajava L.: Taxonomy, Relatives, and possible Origin / Leslie R. Landrum -- Production and Trade / Edward A. Evans and Fredy H.Ballen -- Composition and Processing / Anup K. Bhattacherjee and Dileep. K. Tandon -- Propagation / Sisir Mitra and P.K.Ray -- Biotechnology / Maneesh Mishra, M. Muthukumar and Sandeep Kumar -- Cultivars and crop improvement / Sisir Mitra -- Nutrition and Irrigation / Sisir Mitra -- Orchard Management / Sisir Mitra and P.K.Pathak -- Flowering / Shu-Yen Lin and Po-An Chen -- Fruit set, development and maturation / Rosemary Du Preez -- Physiological disorders / Nor Elliza Tajidin, Munirah Mohamad , Azimah Hamidon, Hamizah Hassan and Siti Hajar Ahmad -- Photosynthesis and productivity / Vinod K. Singh and Manoj K. Soni -- Pests / Rodrigo Lasa, Andrea Birke, Larissa Guillén, Martín Aluja and Daniel Carrillo -- Nematodes / Regina M.D.G. Carneiro, Marcilene F.A. Santos and José Mauro C. Castro -- Diseases / Ashok K. Misra -- Postharvest physiology and storage / Margo Sulistio and Chun-Ta Wu.1 online resource (xii, 368 pages)
R. C. Mitra. The decline of Buddhism in India
No full textFilliozat Jean. R. C. Mitra. The decline of Buddhism in India. In: Revue de l'histoire des religions, tome 149, n°1, 1956. pp. 88-90
Mathematical Complexities in Porous Media Flow
No full textMultiphase flow through porous media plays an important role in many practical applications, from groundwater modelling, oil and gas recovery to CO2 sequestration. In the current work, we address two challenges related to accurate modelling and simulation of such processes. The first is to incorporate non-equilibrium effects such
as hysteresis and dynamic capillarity in the models. Experiments have shown that under certain circumstances, phenomena like saturation overshoot and finger formation
occur, that cannot be explained by the standard (equilibrium) models. Hence, an extension of these models needs to be considered.
The second is to develop fast, stable and preferably simple numerical techniques that solve the highly nonlinear and possibly degenerate equations governing flow in the extremely heterogeneous porous domains of the real world. Accordingly, this
work is divided into two parts:
(PART I: Non-equilibrium effects)
First, a new model is proposed for hysteresis in capillary pressure, which extends and improves the play-type hysteresis model. It is shown that this model is physically
consistent and approximates experimentally obtained hysteresis curves. It is then used to solve the problem of horizontal redistribution of water and air, demonstrating
that ‘unconventional’ flow, predicted earlier in literature, does indeed occur in certain cases. To follow-up, we show that the model is mathematically well-posed. Next, gravity-driven infiltration of water into relatively dry soil is considered
when the wetting front has the form of a downward propagating travelling wave, i.e., the wetting front moves at a constant speed and shape. We consider various cases with increasing complexity in a number of chapters. In the first, we study the
behaviour of fronts when either hysteresis or dynamic capillarity is included. In the second chapter, both effects are included simultaneously and both the play-type and the extended play-type models, mentioned in the previous paragraph, are considered.
The existence of travelling waves is proved and criteria for the occurrence of overshoots and the system to reach full saturation are made precise. The techniques developed are further used in the third chapter to describe viscous fingering and to
derive the propagation speed of the fingers.
Finally, fronts are analysed for the two-phase case in a very general setting where
the relative permeabilities, as well as the capillary pressure, are hysteretic and dynamic capillary effect is included. Existence of all possible travelling wave solutions is shown and a number of qualitative properties are established. The travelling wave
solutions are then used to derive admissibility conditions for shocks in the hyperbolic limit. The entropy solutions derived in this way are much broader compared to the standard entropy solutions of the Buckley-Leverett equation since they can be
non-monotone and have multiple shocks. These results are used to explain experimental observations such as non-monotone saturation profiles and stable saturation plateaus, which were previously not well-understood. (PART II: Numerical methods)
A linear domain decomposition scheme is proposed for heterogeneous and in particular,
layered porous media. Apart from being parallelizable, it is unconditionally convergent for a mild restriction on the time step. Moreover, it is, in general, more
stable and better conditioned than standard monolithic schemes such as the Newton
or the Picard scheme, while being comparable in speed. The issue of nonlinearity is handled in the following chapter where a linear iterative scheme is proposed for solving the nonlinear diffusion equations that arise in
porous flow problems. Being a modified version of the L-scheme, it converges linearly
for a mild restriction on the time step, having convergence rate proportional to
an exponent of the time step size. The convergence is also guaranteed for degenerate
cases. This makes it faster than both the L-scheme and the Picard scheme and more
stable than the Newton and the Picard scheme. Numerical results are provided that
support the analytical findings. Finally, a mixed finite element method is proposed for the two-phase flow model with dynamic capillarity effect. Error estimates are derived showing that the scheme is first order in both space and time. The numerical results support our conclusion.The work in this thesis has been funded by the Royal Dutch Shell and the Netherlands Organisation for Scientific Research (NWO) through the CSER programme (project 14CSER016) and by Hasselt University, Belgium through the project BOF17BL04
Mathematical Complexities in Porous Media Flow
No full textMultiphase flow through porous media plays an important role in many practical applications, from groundwater modelling, oil and gas recovery to CO2 sequestration. In the current work, we address two challenges related to accurate modelling and simulation of such processes. The first is to incorporate non-equilibrium effects such
as hysteresis and dynamic capillarity in the models. Experiments have shown that under certain circumstances, phenomena like saturation overshoot and finger formation
occur, that cannot be explained by the standard (equilibrium) models. Hence, an extension of these models needs to be considered.
The second is to develop fast, stable and preferably simple numerical techniques that solve the highly nonlinear and possibly degenerate equations governing flow in the extremely heterogeneous porous domains of the real world. Accordingly, this
work is divided into two parts:
(PART I: Non-equilibrium effects)
First, a new model is proposed for hysteresis in capillary pressure, which extends and improves the play-type hysteresis model. It is shown that this model is physically
consistent and approximates experimentally obtained hysteresis curves. It is then used to solve the problem of horizontal redistribution of water and air, demonstrating
that ‘unconventional’ flow, predicted earlier in literature, does indeed occur in certain cases. To follow-up, we show that the model is mathematically well-posed. Next, gravity-driven infiltration of water into relatively dry soil is considered
when the wetting front has the form of a downward propagating travelling wave, i.e., the wetting front moves at a constant speed and shape. We consider various cases with increasing complexity in a number of chapters. In the first, we study the
behaviour of fronts when either hysteresis or dynamic capillarity is included. In the second chapter, both effects are included simultaneously and both the play-type and the extended play-type models, mentioned in the previous paragraph, are considered.
The existence of travelling waves is proved and criteria for the occurrence of overshoots and the system to reach full saturation are made precise. The techniques developed are further used in the third chapter to describe viscous fingering and to
derive the propagation speed of the fingers.
Finally, fronts are analysed for the two-phase case in a very general setting where
the relative permeabilities, as well as the capillary pressure, are hysteretic and dynamic capillary effect is included. Existence of all possible travelling wave solutions is shown and a number of qualitative properties are established. The travelling wave
solutions are then used to derive admissibility conditions for shocks in the hyperbolic limit. The entropy solutions derived in this way are much broader compared to the standard entropy solutions of the Buckley-Leverett equation since they can be
non-monotone and have multiple shocks. These results are used to explain experimental observations such as non-monotone saturation profiles and stable saturation plateaus, which were previously not well-understood. (PART II: Numerical methods)
A linear domain decomposition scheme is proposed for heterogeneous and in particular,
layered porous media. Apart from being parallelizable, it is unconditionally convergent for a mild restriction on the time step. Moreover, it is, in general, more
stable and better conditioned than standard monolithic schemes such as the Newton
or the Picard scheme, while being comparable in speed. The issue of nonlinearity is handled in the following chapter where a linear iterative scheme is proposed for solving the nonlinear diffusion equations that arise in
porous flow problems. Being a modified version of the L-scheme, it converges linearly
for a mild restriction on the time step, having convergence rate proportional to
an exponent of the time step size. The convergence is also guaranteed for degenerate
cases. This makes it faster than both the L-scheme and the Picard scheme and more
stable than the Newton and the Picard scheme. Numerical results are provided that
support the analytical findings. Finally, a mixed finite element method is proposed for the two-phase flow model with dynamic capillarity effect. Error estimates are derived showing that the scheme is first order in both space and time. The numerical results support our conclusion.The work in this thesis has been funded by the Royal Dutch Shell and the Netherlands Organisation for Scientific Research (NWO) through the CSER programme (project 14CSER016) and by Hasselt University, Belgium through the project BOF17BL04
On the aberration–retardation effects in pulsars
No full textThe magnetospheric locations of pulsar radio emission region are not well known. The actual form of the so-called radius-to-frequency mapping should be reflected in the aberration-retardation (A/R) effects that shift and/or delay the photons depending on the emission height in the magnetosphere. Recent studies suggest that in a handful of pulsars the A/R effect can be discerned with respect to the peak of the central core emission region. To verify these effects in an ensemble of pulsars, we launched a project analysing multifrequency total intensity pulsar profiles obtained from the new observations from the Giant Meterwave Radio Telescope (GMRT), Arecibo Observatory (AO) and archival European Pulsar Network (EPN) data. For all these profiles, we measure the shift of the outer cone components with respect to the core component, which is necessary for establishing the A/R effect. Within our sample of 23 pulsars, seven show the A/R effects, 12 of them (doubtful cases) show a tendency towards this effect, while the remaining four are obvious counterexamples. The counterexamples and doubtful cases may arise from uncertainties in the determination of the location of the meridional plane and/or the core emission component. Hence, it appears that the A/R effects are likely to operate in most pulsars from our sample. We conclude that in cases where those effects are present the core emission has to originate below the conal emission region
Using mixtures of t densities to make inferences in the presence of missing data with a small number of multiply imputed data sets
No full textStrategies for making inference in the presence of missing data after conducting a Multiple Imputation (MI) procedure are considered. An approach which approximates the posterior distribution for parameters using a mixture of tt-distributions is proposed. Simulated experiments show this approach improves inferences in some aspects, making them more stable over repeated analysis and creating narrower bounds for certain common statistics of interest. Extensions to the existing literature have been executed that provide further stability to inferences and also a strong potential to identify ways to make the analysis procedure more flexible. The competing methods have been first compared using simulated data sets and then a real data set concerning analysis of the effect of breastfeeding duration on children’s cognitive ability. R code to implement the methods used is available as online supplementary material
Appropriate Similarity Measures for Author Cocitation Analysis
No full textWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
MULTIPHONON INFRARED ABSORPTION IN
No full text Supported by U. S. Air Force Cambridge Research Laboratories. R. Marshall and S. S. Mitra, Phys. Rev. (To be published). S. S. Mitra, Phys. Rev. 132, 986, (1963). S. S. Mitra and R. Marshall, Bull. Am. Phys. Soc. 9, 259, (1964).Author Institution: Physics Division, IIT Research InstituteInfrared transmission spectra of ZnO single crystals have been investigated from 500 to , and tentative assignment of the 25 observed absorption maxima to multiphonon processes leads to the following characteristic phonon frequencies (in ) and , belonging to the Brillouin zone boundary. In wurtzite ZnO the degeneracy of the transverse modes breaks down as was also observed in CdS and The assignments were consistent with temperature dependence and polarization of the observed bands. The characteristic phonon frequencies have been discussed in terms of crystal structure and effective ionic The assignment is consistent with the established trends for the II-VI $compounds.^{3}
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