55,279 research outputs found
Evidence for risk of bias in cluster randomised trials: review of recent trials published in three general medical journals
Objective To examine the prevalence of a risk of bias associated with the design and conduct of cluster randomised controlled trials among a sample of recently published studies. Design Retrospective review of cluster randomised trials published in the BMJ, Lancet, and New England Journal of Medicine from January 1997 to October 2002. Main outcome measures Prevalence of secure randomisation of clusters, identification of participants before randomisation (to avoid foreknowledge of allocation), differential recruitment between treatment arms, differential application of inclusion and exclusion criteria, and differential attrition. Results Of the 36 trials identified, 24 were published in the BMJ, I I in the Lancet, and a single trial in the New England journal of Medicine. At the cluster level, 15 (42%) trials provided evidence for secure allocation and 25 (69%) used stratified allocation. Few trials showed evidence of imbalance at the cluster level. However, some evidence of susceptibility to risk of bias at the individual level existed in 14 (39%) studies. Conclusions Some recently published cluster randomised trials may not have taken adequate precautions to guard against threats to the internal validity of their design
The Optimum Distance at which to Determine the Size of a Giant Air Shower
To determine the size of an extensive air shower it is not necessary to have
knowledge of the function that describes the fall-off of signal size from the
shower core (the lateral distribution function). In this paper an analysis with
a simple Monte Carlo model is used to show that an optimum ground parameter can
be identified for each individual shower. At this optimal core distance,
, the fluctuations in the expected signal, ,
due to a lack of knowledge of the lateral distribution function are minimised.
Furthermore it is shown that the optimum ground parameter is determined
primarily by the array geometry, with little dependence on the energy or zenith
angle of the shower or choice of lateral distribution function. For an array
such as the Pierre Auger Southern Observatory, with detectors separated by 1500
m in a triangular configuration, the optimum distance at which to measure this
characteristic signal is close to 1000 m
Hysteresis and nonequilibrium work theorem for DNA unzipping
We study by using Monte Carlo simulations the hysteresis in unzipping and
rezipping of a double stranded DNA (dsDNA) by pulling its strands in opposite
directions in the fixed force ensemble. The force is increased, at a constant
rate from an initial value to some maximum value that lies above
the phase boundary and then decreased back again to . We observed
hysteresis during a complete cycle of unzipping and rezipping. We obtained
probability distributions of work performed over a cycle of unzipping and
rezipping for various pulling rates. The mean of the distribution is found to
be close (the difference being within 10%, except for very fast pulling) to the
area of the hysteresis loop. We extract the equilibrium force versus separation
isotherm by using the work theorem on repeated non-equilibrium force
measurements. Our method is capable of reproducing the equilibrium and the
non-equilibrium force-separation isotherms for the spontaneous rezipping of
dsDNA.Comment: 8 figures, Final version to appear in Physical Review
Analytic solutions of the magnetic annihilation and reconnection problems. I. Planar flow profiles
The phenomena of steady-state magnetic annihilation and reconnection in the vicinity of magnetic nulls are considered. It is shown that reconnective solutions can be derived by superposing the velocity and magnetic fields of simple magnetic annihilation models. These solutions contain most of the previous models for magnetic merging and reconnection, as well as introducing several new solutions. The various magnetic dissipation mechanisms are classified by examining the scaling of the Ohmic diffusion rate with plasma resistivity. Reconnection solutions generally allow more favorable "fast" dissipation scalings than annihilation models. In particular, reconnection models involving the advection of planar field components have the potential to satisfy the severe energy release requirements of the solar flare. The present paper is mainly concerned with magnetic fields embedded in strictly planar flowsâa discussion of the more complicated three-dimensional flow patterns is presented in Part II [Phys. Plasmas 4, 110 (1997)]
Spectra of Maser Radiation from a Turbulent, Circumnuclear Accretion Disk. III. Circular polarization
Calculations are performed for the circular polarization of maser radiation
from a turbulent, Keplerian disk that is intended to represent the sub-parsec
disk at the nucleus of the galaxy NGC4258. The polarization in the calculations
is a result of the Zeeman effect in the regime in which the Zeeman splitting is
much less than the spectral linebreadth. Plausible configurations for turbulent
magnetic and velocity fields in the disk are created by statistical methods.
This turbulence, along with the Keplerian velocity gradients and the blending
of the three hyperfine components to form the masing
transition of water, are key ingredients in determining the appearance of the
polarized spectra that are calculated. These spectra are quite different from
the polarized spectra that would be expected for a two-level transition where
there is no hyperfine structure. The effect of the hyperfine structure on the
polarization is most striking in the calculations for the maser emission that
represents the central (or systemic) features of NGC4258. Information about
magnetic fields is inferred from observations for polarized maser radiation and
bears on the structure of accretion disks.Comment: Latex, uses aastex, eucal, to be published in the Astrophysical
Journa
Hall current effects in dynamic magnetic reconnection solutions
The impact of Hall current contributions on flow driven planar magnetic merging solutions is discussed. The Hall current is important if the dimensionless Hall parameter (or normalized ion skin depth) satisfies cH>η where η is the inverse Lundquist number for the plasma. A dynamic analysis of the problem shows, however, that the Hall current initially manifests itself, not by modifying the planar reconnection field, but by inducing a non-reconnecting perpendicular "separator" component in the magnetic field. Only if the stronger condition c2/H > η is satisfied can Hall currents be expected to affect the planar merging. These analytic predictions are then tested by performing a series of numerical experiments in periodic geometry, using the full system of planar magnetohydrodynamic (MHD) equations. The numerical results confirm that the nature of the merging changes dramatically when the Hall coupling satisfies c2/H > η. In line with the analytic treatment of sheared reconnection, the coupling provided by the Hall term leads to the emergence of multiple current layers that can enhance the global Ohmic dissipation at the expense of the reconnection rate. However, the details of the dissipation depend critically on the symmetries of the simulation, and when the merging is "head-on" (i.e., comprises fourfold symmetry) the reconnection rate can be enhanced
Bessel beam propagation: Energy localization and velocity
The propagation of a Bessel beam (or Bessel-X wave) is analyzed on the basis
of a vectorial treatment. The electric and magnetic fields are obtained by
considering a realistic situation able to generate that kind of scalar field.
Specifically, we analyze the field due to a ring-shaped aperture over a
metallic screen on which a linearly polarized plane wave impinges. On this
basis, and in the far field approximation, we can obtain information about the
propagation of energy flux and the velocity of the energy.Comment: 6 pages, 4 figure
Ultraviolet degradation of thin films of zinc oxide
Ultraviolet degradation of zinc oxide thin film
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