5,887 research outputs found
plink: An R Package for Linking Mixed-Format Tests Using IRT-Based Methods
The R package plink has been developed to facilitate the linking of mixed-format tests for multiple groups under a common item design using unidimensional and multidimensional IRT-based methods. This paper presents the capabilities of the package in the context of the unidimensional methods. The package supports nine unidimensional item response models (the Rasch model, 1PL, 2PL, 3PL, graded response model, partial credit and generalized partial credit model, nominal response model, and multiple-choice model) and four separate calibration linking methods (mean/sigma, mean/mean, Haebara, and Stocking-Lord). It also includes functions for importing item and/or ability parameters from common IRT software, conducting IRT true-score and observed-score equating, and plotting item response curves and parameter comparison plots.
Efficient solutions of self-consistent mean field equations for dewetting and electrostatics in nonuniform liquids
We use a new configuration-based version of linear response theory to
efficiently solve self-consistent mean field equations relating an effective
single particle potential to the induced density. The versatility and accuracy
of the method is illustrated by applications to dewetting of a hard sphere
solute in a Lennard-Jones fluid, the interplay between local hydrogen bond
structure and electrostatics for water confined between two hydrophobic walls,
and to ion pairing in ionic solutions. Simulation time has been reduced by more
than an order of magnitude over previous methods.Comment: Supplementary material included at end of main pape
Ice storm effects on the canopy structure of a northern hardwood forest after 8 years
Ice storms can cause severe damage to forest canopies, resulting in differential mortality among tree species and size classes and leading to long-lasting changes in the vertical structure and composition of the forest. An intense ice storm in 1998 damaged large areas of the northern hardwood forest, including much of the Hubbard Brook Experimental Forest, New Hampshire (USA). Following up on detailed poststorm assessments, we measured changes in the vertical structure of the forest canopy 8 years poststorm. We focused on how the presence of disease-induced advance regeneration of American beech (Fagus grandifolia Ehrh.) has affected canopy structure in the recovering forest. We measured foliage-height profiles using a point-quadrat approach and a pole-mounted leaf area index (LAI) sensor. Although the total LAIs of damaged and undamaged areas were similar, areas damaged in 1998 showed an increased proportion of total leaf area between 6 and 10 m above the ground. The foliage at this height is largely (54%) beech. To the extent that this heavily beech-dominated understory layer suppresses regeneration of other species, these findings suggest that rare disturbances of mature northern hardwood forests affected by beech bark disease will increase the importance of damage-prone and economically marginal beech
A new approach for efficient simulation of Coulomb interactions in ionic fluids
We propose a simplified version of local molecular field (LMF) theory to
treat Coulomb interactions in simulations of ionic fluids. LMF theory relies on
splitting the Coulomb potential into a short-ranged part that combines with
other short-ranged core interactions and is simulated explicitly. The averaged
effects of the remaining long-ranged part are taken into account through a
self-consistently determined effective external field. The theory contains an
adjustable length parameter sigma that specifies the cut-off distance for the
short-ranged interaction. This can be chosen to minimize the errors resulting
from the mean-field treatment of the complementary long-ranged part. Here we
suggest that in many cases an accurate approximation to the effective field can
be obtained directly from the equilibrium charge density given by the Debye
theory of screening, thus eliminating the need for a self-consistent treatment.
In the limit sigma -> 0, this assumption reduces to the classical Debye
approximation. We examine the numerical performance of this approximation for a
simple model of a symmetric ionic mixture. Our results for thermodynamic and
structural properties of uniform ionic mixtures agree well with similar results
of Ewald simulations of the full ionic system. In addition we have used the
simplified theory in a grand-canonical simulation of a nonuniform ionic mixture
where an ion has been fixed at the origin. Simulations using short-ranged
truncations of the Coulomb interactions alone do not satisfy the exact
condition of complete screening of the fixed ion, but this condition is
recovered when the effective field is taken into account. We argue that this
simplified approach can also be used in the simulations of more complex
nonuniform systems.Comment: To be published in Journal of Chemical Physic
Environmental legislation as a driver of design
and other research output
Impact of a Professional Development Experience Focused on Extension Educators as Change Agents
Extension educators should think of themselves as change agents, yet many act as information dispensers. Accordingly, we sought to determine whether we could change perceptions of county Extension educators in Oklahoma regarding their function as change agents. Educators participated in a two-part (two-treatment) professional development experience involving a workshop and a computer simulation. The experience was grounded in Rogers\u27s diffusion of innovations theory and addressed nine established Extension educator change-agent roles. Participants ranked the importance of the roles prior to the experience and again following each treatment. Their perceptions changed regarding only the Alternative Delivery Systems role. We explore the imperative to improve Extension agents\u27 understanding of how to be change agents
Density fluctuations and the structure of a nonuniform hard sphere fluid
We derive an exact equation for density changes induced by a general external
field that corrects the hydrostatic approximation where the local value of the
field is adsorbed into a modified chemical potential. Using linear response
theory to relate density changes self-consistently in different regions of
space, we arrive at an integral equation for a hard sphere fluid that is exact
in the limit of a slowly varying field or at low density and reduces to the
accurate Percus-Yevick equation for a hard core field. This and related
equations give accurate results for a wide variety of fields
Properties of cage rearrangements observed near the colloidal glass transition
We use confocal microscopy to study the motions of particles in concentrated
colloidal systems. Near the glass transition, diffusive motion is inhibited, as
particles spend time trapped in transient ``cages'' formed by neighboring
particles. We measure the cage sizes and lifetimes, which respectively shrink
and grow as the glass transition approaches. Cage rearrangements are more
prevalent in regions with lower local concentrations and higher disorder.
Neighboring rearranging particles typically move in parallel directions,
although a nontrivial fraction move in anti-parallel directions, usually from
pairs of particles with initial separations corresponding to the local maxima
and minima of the pair correlation function , respectively.Comment: 5 pages, 4 figures; text & figures revised in v
Local molecular field theory for the treatment of electrostatics
We examine in detail the theoretical underpinnings of previous successful
applications of local molecular field (LMF) theory to charged systems. LMF
theory generally accounts for the averaged effects of long-ranged components of
the intermolecular interactions by using an effective or restructured external
field. The derivation starts from the exact Yvon-Born-Green hierarchy and shows
that the approximation can be very accurate when the interactions averaged over
are slowly varying at characteristic nearest-neighbor distances. Application of
LMF theory to Coulomb interactions alone allows for great simplifications of
the governing equations. LMF theory then reduces to a single equation for a
restructured electrostatic potential that satisfies Poisson's equation defined
with a smoothed charge density. Because of this charge smoothing by a Gaussian
of width sigma, this equation may be solved more simply than the detailed
simulation geometry might suggest. Proper choice of the smoothing length sigma
plays a major role in ensuring the accuracy of this approximation. We examine
the results of a basic confinement of water between corrugated wall and justify
the simple LMF equation used in a previous publication. We further generalize
these results to confinements that include fixed charges in order to
demonstrate the broader impact of charge smoothing by sigma. The slowly-varying
part of the restructured electrostatic potential will be more symmetric than
the local details of confinements.Comment: To be published in J Phys-Cond Matt; small misprint corrected in Eq.
(12) in V
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