Adaptive multigrid algorithm for the lattice Wilson-Dirac operator

We present an adaptive multigrid solver for application to the non-Hermitian Wilson-Dirac system of QCD. The key components leading to the success of our proposed algorithm are the use of an adaptive projection onto coarse grids that preserves the near null space of the system matrix together with a simplified form of the correction based on the so-called gamma_5-Hermitian symmetry of the Dirac operator. We demonstrate that the algorithm nearly eliminates critical slowing down in the chiral limit and that it has weak dependence on the lattice volume

Application of block Krylov subspace algorithms to the Wilson-Dirac equation with multiple right-hand sides in lattice QCD

It is well known that the block Krylov subspace solvers work efficiently for some cases of the solution of differential equations with multiple right-hand sides. In lattice QCD calculation of physical quantities on a given configuration demands us to solve the Dirac equation with multiple sources. We show that a new block Krylov subspace algorithm recently proposed by the authors reduces the computational cost significantly without loosing numerical accuracy for the solution of the O(a)-improved Wilson-Dirac equation.Comment: 12 pages, 5 figure

First-Order System Least Squares and the Energetic Variational Approach for Two-Phase Flow

This paper develops a first-order system least-squares (FOSLS) formulation for equations of two-phase flow. The main goal is to show that this discretization, along with numerical techniques such as nested iteration, algebraic multigrid, and adaptive local refinement, can be used to solve these types of complex fluid flow problems. In addition, from an energetic variational approach, it can be shown that an important quantity to preserve in a given simulation is the energy law. We discuss the energy law and inherent structure for two-phase flow using the Allen-Cahn interface model and indicate how it is related to other complex fluid models, such as magnetohydrodynamics. Finally, we show that, using the FOSLS framework, one can still satisfy the appropriate energy law globally while using well-known numerical techniques.Comment: 22 pages, 8 figures submitted to Journal of Computational Physic

Investigating possible ethnicity and sex bias in clinical examiners: an analysis of data from the MRCP(UK) PACES and nPACES examinations

Bias of clinical examiners against some types of candidate, based on characteristics such as sex or ethnicity, would represent a threat to the validity of an examination, since sex or ethnicity are 'construct-irrelevant' characteristics. In this paper we report a novel method for assessing sex and ethnic bias in over 2000 examiners who had taken part in the PACES and nPACES (new PACES) examinations of the MRCP(UK)

What is Method Variance and How Can We Cope With It? A Panel Discussion

A panel of experts describes the nature of, and remedies for, method variance. In an attempt to help the reader understand the nature of method variance, the authors describe their experiences with method variance both on the giving and the receiving ends of the editorial review process, as well as their interpretation of other reviewers’ comments. They then describe methods of data analysis and research design, which have been used for detecting and eliminating the effects of method variance. Most methods have some utility, but none prevent the researcher from making faulty inferences. The authors conclude with suggestions for resolving disputes about method variance

Extending the applicability of multigrid methods

Multigrid methods are ideal for solving the increasingly large-scale problems that arise in numerical simulations of physical phenomena because of their potential for computational costs and memory requirements that scale linearly with the degrees of freedom. Unfortunately, they have been historically limited by their applicability to elliptic-type problems and the need for special handling in their implementation. In this paper, we present an overview of several recent theoretical and algorithmic advances made by the TOPS multigrid partners and their collaborators in extending applicability of multigrid methods. Specific examples that are presented include quantum chromodynamics, radiation transport, and electromagnetics

Does changing examiner stations during UK postgraduate surgery objective structured clinical examinations influence examination reliability and candidates' scores?

Objective Objective structured clinical examinations (OSCE) are widely used for summative assessment in surgery. Despite standardizing these as much as possible, variation, including examiner scoring, can occur which may affect reliability. In study of a high-stakes UK postgraduate surgical OSCE, we investigated whether examiners changing stations once during a long examining day affected marking, reliability, and overall candidates’ scores compared with examiners who examined the same scenario all day. Design, Setting, and Participants An observational study of 18,262 examiner-candidate interactions from the UK Membership of the Royal College of Surgeons examination was carried at 3 Surgical Colleges across the United Kingdom. Scores between examiners were compared using analysis of variance. Examination reliability was assessed with Cronbach’s alpha, and the comparative distribution of total candidates’ scores for each day was evaluated using t-tests of unit-weighted z scores. Results A significant difference was found in absolute scores differences awarded in the morning and afternoon sessions between examiners who changed stations at lunchtime and those who did not (p < 0.001). No significant differences were found for the main effects of either broad content area (p = 0.290) or station content area (p = 0.450). The reliability of each day was not affected by examiner switching (p = 0.280). Overall, no difference was found in z-score distribution of total candidate scores and categories of examiner switching. Conclusions This large study has found that although the range of marks awarded varied when examiners change OSCE stations, examination reliability and the likely candidate outcome were not affected. These results may have implications for examination design and examiner experience in surgical OSCEs and beyond