Multiple imputation of multiple multi-item scales when a full imputation model is infeasible.

BACKGROUND: Missing data in a large scale survey presents major challenges. We focus on performing multiple imputation by chained equations when data contain multiple incomplete multi-item scales. Recent authors have proposed imputing such data at the level of the individual item, but this can lead to infeasibly large imputation models. METHODS: We use data gathered from a large multinational survey, where analysis uses separate logistic regression models in each of nine country-specific data sets. In these data, applying multiple imputation by chained equations to the individual scale items is computationally infeasible. We propose an adaptation of multiple imputation by chained equations which imputes the individual scale items but reduces the number of variables in the imputation models by replacing most scale items with scale summary scores. We evaluate the feasibility of the proposed approach and compare it with a complete case analysis. We perform a simulation study to compare the proposed method with alternative approaches: we do this in a simplified setting to allow comparison with the full imputation model. RESULTS: For the case study, the proposed approach reduces the size of the prediction models from 134 predictors to a maximum of 72 and makes multiple imputation by chained equations computationally feasible. Distributions of imputed data are seen to be consistent with observed data. Results from the regression analysis with multiple imputation are similar to, but more precise than, results for complete case analysis; for the same regression models a 39% reduction in the standard error is observed. The simulation shows that our proposed method can perform comparably against the alternatives. CONCLUSIONS: By substantially reducing imputation model sizes, our adaptation makes multiple imputation feasible for large scale survey data with multiple multi-item scales. For the data considered, analysis of the multiply imputed data shows greater power and efficiency than complete case analysis. The adaptation of multiple imputation makes better use of available data and can yield substantively different results from simpler techniques

Scaling in Plasticity-Induced Cell-Boundary Microstructure: Fragmentation and Rotational Diffusion

We develop a simple computational model for cell boundary evolution in plastic deformation. We study the cell boundary size distribution and cell boundary misorientation distribution that experimentally have been found to have scaling forms that are largely material independent. The cell division acts as a source term in the misorientation distribution which significantly alters the scaling form, giving it a linear slope at small misorientation angles as observed in the experiments. We compare the results of our simulation to two closely related exactly solvable models which exhibit scaling behavior at late times: (i) fragmentation theory and (ii) a random walk in rotation space with a source term. We find that the scaling exponents in our simulation agree with those of the theories, and that the scaling collapses obey the same equations, but that the shape of the scaling functions depend upon the methods used to measure sizes and to weight averages and histograms

Sintered Bottom and Vitrified Silica Ashes Derived from Incinerated Municipal Solid Waste as Circular Economy-Friendly Partial Replacements for Cement in Mortars

Sintered bottom ash (SBA) and vitrified ash (VA) derived from the incineration of municipal solid waste (MSW) at 1200 °C were used in this study as replacements for Type 2 Portland cement (CEM-II) based mortars. This approach negates the need to send them to landfill, benefits the circular economy and contributes towards the decarbonisation of cementitious construction materials in response to international net zero carbon emission agendas. The material (physico-chemical) characteristics of VA and SBA were analysed before being used as partial replacements for CEM-II in mortars, whereby compressive strength (CS) was the primary criterion for assessing engineering performance. VA and SBA replaced CEM-II at dosages of 10%, 25% and 50% based on their high inorganic and pozzolanic contents; whereby the 10% and 25% replacements did not compromise mortar strength. The alkalinity and pozzolanic properties of SBA collectively indicated it has greater potential as a cementitious material over VA, which possessed a neutral pH. The 28-day CS recorded for mixtures containing 25% VA and 10% SBA were 13.74 MPa and 11.77 MPa, respectively compared with 17.06 MPa for CEM-II control samples. The use of 2% additional water in 25% SBA mortar designs improved strength further, indicating that SBA’s water retention properties permitted further hydration and strength development with curing. Microstructural, mineralogical and infrared spectroscopy analyses indicated that these strengths were owed to the formation of silicate-based hydration products. The outcomes from this study highlight that SBA has potential for replacing CEM-II and VA as a filler in cementitious mortar. Graphical Abstract: [Figure not available: see fulltext.]</p

The significance of work allocation in the professional apprenticeship of solicitors

It is a peculiarity of the solicitors’ profession that it has historically relied on methods of pre-qualification ‘training’ by way of apprenticeship and that an entirely respectable non-graduate route into the profession remains. In a political context, however, where the profession is called upon positively to demonstrate its standards of performance, the professional regulator seeks to attach a competence framework to the existing model; shifting the focus from how the trainee learns to what the trainee learns. This paper will explore the period of traineeship from the perspective of the trainees themselves, drawing on two small qualitative studies, focussing on the fundamental context factor of the allocation and structuring of their work. In the first study the context for this evaluation is the set of outcomes being tested by the professional regulator and in the second, the perceptions of qualified individuals looking back at their apprenticeship, The paper concludes that there remains work for the profession to do not only in fostering supportive and expansive apprenticeships, but in attending, however, supportive the surrounding environment, to the work being carried out by trainees and its relationship with the work carried out by newly qualified solicitors

First steps in the development of a water temperature model framework for refining the ecological Reserve in South African rivers

Ecological Reserve determination for rivers in South Africa presently does not include a water temperature component, in spite of its importance in determining species distribution patterns. To achieve this requires an understanding of how lotic thermographs from South African rivers differ from northern hemisphere rivers, to avoid mismanaging rivers based on incorrect regional assumptions. Hourly water temperatures from 20 sites in four river systems, representing a range of latitudes, altitudes and stream orders, were assessed using a range of metrics. These data were analysed using principal component analyses and multiple linear regressions to understand what variables a water temperature model for use in ecoregions within South Africa should include. While temperature data are generally lacking in low- and higher-order South African rivers, data suggest that South African rivers are warmer than northern hemisphere rivers. Water temperatures could be grouped into cool, warm and intermediate types. Based on temperature time series analyses, this paper argues that a suitable water-temperature model for use in ecological Reserve determinations should be dynamic, include flow and air temperature variables, and be adaptive through a heat exchange coefficient term. The inclusion of water temperature in the determination and management of river ecological Reserves would allow for more holistic application of the National Water Act’s ecological management provisions. Water temperature guidelines added to the ecological Reserve could be integrated into heuristic aquatic monitoring programmes within priority areas identified in regional conservation plans.Keywords: water temperatures, conservation planning, water temperature modelling, managemen

Observations and Empirical Scalings of the High-Confinement Mode Pedestal on Alcator C-Mod

and disposal, in whole or in part, by or for the United States government is permitted. Observations and empirical scalings of the high-confinement mode pedestal on Alcator C-Mod J.W. Hughes, D.A. Mossessian, A.E. Hubbard, B. LaBombard, E.S. Marmar On the Alcator C-Mod tokamak [Phys. Plasmas 1, 1511, (1994)], radial pro-files of electron temperature (Te) and density (ne) are measured at the plasma edge with millimeter resolution Thomson scattering [Rev. Sci. Instrum. 72, 1107 (2001)]. Edge transport barriers in the high confinement regime (H-mode) exhibit Te, ne pedestals with typical widths of 2–6 mm, with the Te pedestal on average slightly wider than and inside the ne pedestal. Measure-ments at both the top and the base of the pedestal are consistent with profiles obtained using other diagnostics. The two primary H-mode regimes on C-Mod, enhanced Dα (EDA) and edge-localized mode free (ELM-free), have been ex-amined for differences in pedestals. EDA operation is favored by high edge collisionality ν, in addition to high edge safety factor q95. Scaling studies at fixed shape yield little systematic variation of pedestal widths with plasma parameters, though higher triangularity is seen to increase the ne pedestal width dramatically. Pedestal heights and gradients show the clearest depen-dencies on plasma control parameters. Pedestal ne and Te both scale linearly with plasma current IP, while pedestal Te depends strongly on power flowing from the core plasma into the scrape-off layer PSOL. The electron pressure (pe) pedestal and pe gradient both scale with I 2 P

Experimental evidence of a metal-insulator transition in a half-filled Landau level

We have measured the low-temperature transport properties of a high-mobility front-gated GaAs/Al_{0.33}Ga_{0.67}As heterostructure. By changing the applied gate voltage, we can vary the amount of disorder within the system. At a Landau level filling factor $\nu =1/2$, where the system can be described by the composite fermion picture, we observe a crossover from metallic to insulating behaviour as the disorder is increased. Experimental results and theoretical prediction are compared.Comment: To be published in Solid State Communications. 4 figure