695915 research outputs found
Sort by
The Magnetic Field in the Milky Way Filamentary Bone G47
No full textStar formation primarily occurs in laments where magnetic fields are expected to be dynamicallyimportant. The largest and densest laments trace spiral structure within galaxies. Over a dozen ofthese dense (∼10 4 cm-3) and long (>10 pc) laments have been found within the Milky Way, and they are often referred to as 'bones." Until now, none of these bones have had their magnetic field resolved and mapped in their entirety. We introduce the SOFIA legacy project FIELDMAPS which has begun mapping ∼10 of these Milky Way bones using the HAWC+ instrument at 214 μm and 18:002 resolution. Here we present a first result from this survey on the ∼60 pc long bone G47. Contrary to some studies of dense laments in the Galactic plane, we find that the magnetic field is often not perpendicular to the spine (i.e., the center-line of the bone). Fields tend to be perpendicular in the densest areas of active star formation and more parallel or random in other areas. The average field is neither parallel or perpendicular to the Galactic plane nor the bone. The magnetic field strengths along the spine typically vary from ∼20 to ∼100 μG. Magnetic fields tend to be strong enough to suppress collapse along much of the bone, but for areas that are most active in star formation, the fields are notably less able to resist gravitational collapse.<br/
Are we leaving money on the table in infertility RCTs? Trialists should statistically adjust for prespecified, prognostic covariates to increase power.
No full textInfertility RCTs are often too small to detect realistic treatment effects. Large observational studies have been proposed as a solution. However, this strategy threatens to weaken the evidence base further, because non-random assignment to treatments makes it impossible to distinguish effects of treatment from confounding factors. Alternative solutions are required. Power in an RCT can be increased by adjusting for prespecified, prognostic covariates when performing statistical analysis, and if stratified randomization or minimization has been used, it is essential to adjust in order to get the correct answer. We present data showing that this simple, free, and frequently necessary strategy for increasing power is seldom employed, even in trials appearing in leading journals. We use this to motivate a pedagogical discussion and provide a worked example. While covariate adjustment can’t solve the problem of underpowered trials outright, there is an imperative to use sound methodology to maximize the information each trial yields
Fairness-Aware Data Integration
No full textMachine learning can be applied in applications that take decisions that impact people’s lives. Such techniques have the potential to make decision making more objective, but there also is a risk that the decisions can discriminate against certain groups as a result of bias in the underlying data. Reducing bias, or promoting fairness, has been a focus of significant investigation in machine learning, for example based on pre-processing the training data, changing the learning algorithm, or post-processing the results of the learning. However, prior to these activities, data integration discovers and integrates the data that is used for training, and data integration processes have the potential to produce data that leads to biased conclusions. In this paper, we propose an approach that generates schema mappings in ways that take into account: (i) properties that are intrinsic to mapping results that may give rise to bias in analyses; and (ii) bias observed in classifiers trained on the results of different sets of mappings. The approach explores a space of different ways of integrating the data, using a tabu search algorithm, guided by bias-aware objective functions that represent different types of bias.The resulting approach is evaluated using Adult Census and German Credit datasets, to explore the extent to which and the circumstances in which the approach can increase the fairness of the results of the data integration process
Towards an atomistic understanding of polymorphism in molecular solids
No full textAbstract Understanding and controlling polymorphism in molecular solids is a major unsolved problem in crystal engineering. While the ability to calculate accurate lattice energies with atomistic modelling provides valuable insight into the associated energy scales, existing methods cannot connect energy differences to the delicate balances of intra- and intermolecular forces that ultimately determine polymorph stability ordering. We report herein a protocol for applying Quantum Chemical Topology (QCT) to study the key intra- and intermolecular interactions in molecular solids, which we use to compare the three known polymorphs of succinic acid including the recently-discovered γ form. QCT provides a rigorous partitioning of the total energy into contributions associated with topological atoms, and a quantitative and chemically intuitive description of the intra- and intermolecular interactions. The newly-proposed Relative Energy Gradient (REG) method ranks atomistic energy terms (steric, electrostatic and exchange) by their importance in constructing the total energy profile for a chemical process. We find that the conformation of the succinic acid molecule is governed by a balance of large and opposing electrostatic interactions, while the H-bond dimerisation is governed by a combination of electrostatics and sterics. In the solids, an atomistic energy balance emerges that governs the contraction, towards the equilibrium geometry, of a molecular cluster representing the bulk crystal. The protocol we put forward is as general as the capabilities of the underlying quantum-mechanical model and it can provide novel perspectives on polymorphism in a wide range of chemical systems.<br/
Risk Assessment of Cascading Failures in Power Systems with Increasing Wind Penetration
No full textThe ever-growing penetration of renewable based gen-eration is leading to significant increases in the risk of cascading failures in low-inertia, and interconnection-rich, power systems. This paper proposes a framework for quantifying the risk of cas-cading failures in renewable-rich power systems with fast fre-quency response (FFR) services. This is achieved by developing a novel time-based dynamic model of cascading failures to capture the transient behaviours of the system response and quantify the risk of blackout. This dynamic cascading failure model is comple-mented by unit commitment for representative dispatch and in-cludes the provision of FFR. The proposed framework is illus-trated using a modified version of the Illinois 200-bus synthetic system. As key outputs, the results emphasize the importance of accurate modelling of ancillary services associated with fre-quency regulation in cascading failure analysis, and indicate that increased wind penetration can lead to a higher probability of power outages. Adopting FFR services can help to mitigate the cascading risk but may introduce rotor angle instability issues
Bounds for the chi-square approximation of the power divergence family of statistics
No full textIt is well-known that each statistic in the family of power divergence statistics, across n trials and r classifications with index parameter λ ∈ R (the Pearson, likelihood ratio and Freeman-Tukey statistics correspond to λ = 1, 0, −1/2, respectively) is asymptotically chi-square distributed as the sample size tends to infinity. In this paper, we obtain explicit bounds on this distributional approximation, measured using smooth test functions, that hold for a given finite sample n, and all index parameters (λ > −1) for which such finite sample bounds are meaningful. We obtain bounds that are of the optimal order n¯¹. The dependence of our bounds on the index parameter λ and the cell classification probabilities is also optimal, and the dependence on the number of cells is also respectable. Our bounds generalise, complement and improve on recent results from the literature
Numerical Stability of Algorithms at Extreme Scale and Low Precisions*
No full textThe largest dense linear systems that are being solved today are of order 𝑛 = 107. Single precision arithmetic, which has a unit roundoff 𝑢 ≈ 10-8, is widely used in scientific computing, and half precision arithmetic, with 𝑢 ≈ 10−4, is increasingly being exploited as it becomes more readily available in hardware. Standard rounding error bounds for numerical linear algebra algorithms are proportional to 𝑝(𝑛)𝑢, with 𝑝 growing at least linearly with 𝑛. Therefore we are at the stage where these rounding error bounds are not able to guarantee any accuracy or stability in the computed results for some extreme-scale or low-accuracy computations. We explain how rounding error bounds with much smaller constants can be obtained. Blocked algorithms, which break the data into blocks of size 𝑏, lead to a reduction in the error constants by a factor 𝑏 or more. Two architectural features also reduce the error constants: extended precision registers and fused multiply–add operations, either at the scalar level or in mixed precision block form. We also discuss a new probabilistic approach to rounding error analysis that provides error constants that are the square roots of those of the worst-case bounds. Combining these different considerations provides new understanding of the numerical stability of extreme scale and low precision computations in numerical linear algebra.<br/
The role of solvation in proton transfer reactions: implications for predicting salt/co-crystal formation using the ΔpKa rule
No full textThe ΔpKa rule is commonly applied by chemists and crystal engineers as a guideline for the rational design of molecular salts and co-crystals. For multi-component crystals containing acid and base constituents, empirical evidence has shown that ΔpKa > 4 almost always leads to salts, ΔpKa < –1 almost always leads to co-crystals and ΔpKa between –1 and 4 can be either. This paper reviews the theoretical background of the ΔpKa rule and highlights the crucial role of solvation in determining the outcome of the potential proton transfer from acid to base. New data on the frequency of occurrence of co-crystals and salts in multi-component crystal structures containing acid and base constituents show that the relationship between ΔpKa and the frequency of salt/co-crystal formation is influenced by the composition of the crystal. For unsolvated co-crystals/salts, containing only the principal acid and base components, the point of 50% probability for salt/co-crystal formation occurs at ΔpKa ≈ 1.4, while for hydrates of co-crystals and salts, this point is shifted to ΔpKa ≈ –0.5. For acid-base crystals with the possibility for two proton transfers, the overall frequency of occurrence of any salt (monovalent or divalent) versus a co-crystal is comparable to that of the whole data set, but the point of 50% probability for observing a monovalent salt vs a divalent salt lies at ΔpKa,II ≈ –4.5. Hence, where two proton transfers are possible, the balance is between co-crystals and divalent salts, with monovalent salts being far less common. Finally, the overall role played by the “crystal” solvation is illustrated by the fact that acid-base complexes in the intermediate region of ΔpKa tip towards salt formation if ancillary hydrogen bonds can exist. Thus, the solvation strength of the lattice plays a key role in the stabilisation of the ions
Understanding the Effect of Water Chemistry and Surface Grinding on the Stress Corrosion Cracking Initiation of Filler Metal 82
No full textStress corrosion cracking (SCC) initiation of specimens extracted from filler metal (FM) 82 weldments was investigated in high purity 288˚C water. Accelerated autoclave slow strain rate tensile (SSRT) tests were conducted at different electrochemical corrosion potentials (ECPs) by varying the water chemistry through adjusting the concentration of dissolved hydrogen and oxygen in the recirculating water. The effect of surface preparation was also investigated. Statistical characterization of SCC initiation results showed that the ground surfaces (using P600 grit) of FM 82 appeared to be more resistant to SCC initiation than the surfaces polished via oxide polishing suspension (OPS). The effect of ECP on the SCC initiation susceptibility is also discussed through statistical characterization of the observed IG cracks
Establishing the reliability and the validity of the Arabic translated versions of the Effort Assessment Scale and the Fatigue Assessment Scale
No full textObjective: The aim was to establish the reliability and the validity of Arabic translated versions of the Fatigue Assessment Scale (FAS) and the Effort Assessment Scale (EAS). Design: The FAS and the EAS were translated from the original English following a recommended six-step approach for translating hearing-related questionnaires for different languages. The reliability of the scales was investigated using Cronbach’s alpha, item-total correlation, and inter-item correlation. Construct validity was investigated using factor analysis and the hypothesis testing method.Study sample: The translated scales were completed by 146 participants from Jordan and Saudi Arabia (age range 19-86 years old, 39% male). Participants’ hearing level ranged from normal to profound.Results: Item 3 in the translated FAS was removed to improve the scale’s construct validity. The translated version of the EAS was found to be as reliable and valid as the original EAS.Conclusion: The availability of standardized versions of the FAS and the EAS provides a quick and easy method for improving hearing rehabilitation in Arabic-speaking countries where audiology services can often be costly and not necessarily accessible to all individuals