1,079 research outputs found
Derivation and assessment of risk prediction models using case-cohort data
Background
Case-cohort studies are increasingly used to quantify the association of novel factors with disease risk. Conventional measures of predictive ability need modification for this design. We show how Harrell’s C-index, Royston’s D, and the category-based and continuous versions of the net reclassification index (NRI) can be adapted.
Methods
We simulated full cohort and case-cohort data, with sampling fractions ranging from 1% to 90%, using covariates from a cohort study of coronary heart disease, and two incidence rates. We then compared the accuracy and precision of the proposed risk prediction metrics.
Results
The C-index and D must be weighted in order to obtain unbiased results. The NRI does not need modification, provided that the relevant non-subcohort cases are excluded from the calculation. The empirical standard errors across simulations were consistent with analytical standard errors for the C-index and D but not for the NRI. Good relative efficiency of the prediction metrics was observed in our examples, provided the sampling fraction was above 40% for the C-index, 60% for D, or 30% for the NRI. Stata code is made available.
Conclusions
Case-cohort designs can be used to provide unbiased estimates of the C-index, D measure and NRI
Antisymmetry and channel coupling contributions to the absorption for p + alpha/d + 3He
To understand recently established empirical p + alpha potentials, RGM
calculations followed by inversion are made to study contributions of the d +
3He reaction channels and deuteron distortion effects to the p + alpha
potential. An equivalent study of the d + 3He potential is also presented. The
contributions of exchange non-locality to the absorption are simulated by
including an phenomenological imaginary potential in the RGM. These effects
alone strongly influence the shape of the imaginary potentials for both p +
alpha and d + 3He. The potentials local-equivalent to the fully
antisymmetrised-coupled channels calculations have a significant
parity-dependence in both real and imaginary components, which for p + alpha is
qualitatively similar to that found empirically. The effects on the potentials
of the further inclusion of deuteron distortion are also presented. The
inclusion of a spin-orbit term in the RGM, adds additional terms to the
phase-equivalent potential, most notably the comparatively large imaginary
spin-orbit term found empirically.Comment: 17 pages, Latex, 8 postscript figs, submitted to Nucl. Phys.
Deuteron -- interaction by inversion of RGM S-matrix: determination of spin-orbit potential for spin-1 projectile
The iterative-perturbative (IP) procedure for S-matrix to potential inversion
is applied to spin-one projectiles for the restricted case of vector spin-orbit
interaction only. In order to evaluate this extension of IP inversion we have
inverted the multi-channel RGM of Kanada et al for deuterons
scattering from He with deuteron distortion and then compared the central
components with those derived from RGM with spin set to zero. Attention is
given to the question of how well the resulting potentials are established.
Reliable spin-1 inversion is demonstrated. Results relating to inversion, to
deuteron-nucleus interactions and to RGM are presented and suggest the range of
nuclear interaction information which the procedure makes possible. Unusual
non-locality and parity dependence effects are found; these are of possible
relevance to generic properties of nuclear potentials.Comment: 15 pages LaTeX plus 6 postscript figure
Information Presentation
The goal of the Information Presentation Directed Research Project (DRP) is to address design questions related to the presentation of information to the crew. The major areas of work, or subtasks, within this DRP are: 1) Displays, 2) Controls, 3) Electronic Procedures and Fault Management, and 4) Human Performance Modeling. This DRP is a collaborative effort between researchers at Johnson Space Center and Ames Research Center
Search for Possible Variation of the Fine Structure Constant
Determination of the fine structure constant alpha and search for its
possible variation are considered. We focus on a role of the fine structure
constant in modern physics and discuss precision tests of quantum
electrodynamics. Different methods of a search for possible variations of
fundamental constants are compared and those related to optical measurements
are considered in detail.Comment: An invited talk at HYPER symposium (Paris, 2002
Effective Electromagnetic Lagrangian at Finite Temperature and Density in the Electroweak Model
Using the exact propagators in a constant magnetic field, the effective
electromagnetic Lagrangian at finite temperature and density is calculated to
all orders in the field strength B within the framework of the complete
electroweak model, in the weak coupling limit. The partition function and free
energy are obtained explicitly and the finite temperature effective coupling is
derived in closed form. Some implications of this result, potentially
interesting to astrophysics and cosmology, are discussed.Comment: 14 pages, Revtex
Morphological suitability for endovascular repair, non-intervention rates, and operative mortality in women and men assessed for intact abdominal aortic aneurysm repair: systematic reviews with meta-analysis
Genetic relationships within and among Iberian fescues (Festuca L.) based on PCR-amplified markers
The genus Festuca comprises approximately 450 species and is widely distributed around the world. The Iberian Penninsula, with more than 100 taxa colonizing very diverse habitats, is one of its main centers of diversification. This study was conducted to assess molecular genetic variation and genetic relatedness among 91 populations of 31 taxa of Iberian fescues, based on several molecular markers (random amplified polymorphic DNA, amplified fragment length polymorphisms, and trnL sequences). The analyses showed the paraphyletic origin of the broad-leaved (subgenus Festuca, sections Scariosae and Subbulbosae, and subgenus Schedonorus) and the fine-leaved fescues (subgenus Festuca, sections Aulaxyper, Eskia, and Festuca). Schedonorus showed a weak relationship with Lolium rigidum and appeared to be the most recent of the broad-leaved clade. Section Eskia was the most ancient and Festuca the most recent of the fine-leaved clade. Festuca and Aulaxyper were the most related sections, in concordance with their taxonomic affinities. All taxa grouped into their sections, except F. ampla and F. capillifolia (section Festuca), which appeared to be more closely related to Aulaxyper and to a new independent section, respectively. Most populations clustered at the species level, but some subspecies and varieties mixed their populations. This study demonstrated the value in combining different molecular markers to uncover hidden genetic relationships between populations of Festuca
Measurement of the tensor analyzing power T20 in the dd->^3Hen and dd->^3Hp at intermediate energies and at zero degree
The data on the tensor analyzing power T20 in the dd->^3Hen and dd-> ^3Hp
reactions at 140, 200 and 270 MeV of the deuteron kinetic energy and at zero
degree obtained at RIKEN Accelerator Research Facility are presented. The
observed positive sign of T20 clearly demonstrates the sensitivity to the D/S
wave ratios in the ^3He and ^3H in the energy domain of the measurements. The
T20 data for the ^3He-n and ^3H-p channels are in agreement within experimental
accuracy.Comment: 9 pages, 3 figures, submitted in Phys.Lett.
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