13,156 research outputs found
How many operating rooms are needed to manage non-elective surgical cases? A Monte Carlo simulation study.
BackgroundPatients often wait to have urgent or emergency surgery. The number of operating rooms (ORs) needed to minimize waiting time while optimizing resources can be determined using queuing theory and computer simulation. We developed a computer program using Monte Carlo simulation to determine the number of ORs needed to minimize patient wait times while optimizing resources.MethodsWe used patient arrival data and surgical procedure length from our institution, a tertiary-care academic medical center that serves a large diverse population. With ~4800 patients/year requiring non-elective surgery, and mean procedure length 185 min (median 150 min) we determined the number of ORs needed during the day and evening (0600-2200) and during the night (2200-0600) that resulted in acceptable wait times.ResultsSimulation of 4 ORs at day/evening and 3 ORs at night resulted in median wait time = 0 min (mean = 19 min) for emergency cases requiring surgery within 2 h, with wait time at the 95th percentile = 109 min. Median wait time for urgent cases needing surgery within 8-12 h was 34 min (mean = 136 min), with wait time at the 95th percentile = 474 min. The effect of changes in surgical length and volume on wait times was determined with sensitivity analysis.ConclusionsMonte Carlo simulation can guide decisions on how to balance resources for elective and non-elective surgical procedures
Swift observations of the 2006 outburst of the recurrent nova RS Ophiuchi: II. 1D hydrodynamical models of wind driven shocks
Following the early Swift X-ray observations of the latest outburst of the
recurrent nova RS Ophiuchi in February 2006 (Paper I), we present new 1D
hydrodynamical models of the system which take into account all three phases of
the remnant evolution. The models suggest a novel way of modelling the system
by treating the outburst as a sudden increase then decrease in wind mass-loss
rate and velocity. The differences between this wind model and previous
Primakoff-type simulations are described. A more complex structure, even in 1D,
is revealed through the presence of both forward and reverse shocks, with a
separating contact discontinuity. The effects of radiative cooling are
investigated and key outburst parameters such as mass-loss rate, ejecta
velocity and mass are varied. The shock velocities as a function of time are
compared to the ones derived in Paper I. We show how the manner in which the
matter is ejected controls the evolution of the shock and that for a
well-cooled remnant, the shock deceleration rate depends on the amount of
energy that is radiated away.Comment: 9 pages, 5 figure
The multifrequency behaviour of the recurrent nova RS Ophiuchi
This review concentrates on the multifrequency behaviour of RS Ophiuchi and
in particular during its latest outburst. Confirmation of the 1945 outburst,
bipolar outflows and its possible fate as a Type Ia Supernova are discussed.Comment: 5 pages, 5 figures, in The Golden Age of Cataclysmic Variables and
Related Objects, F. Giovannelli & L. Sabau-Graziati (eds.), Mem. SAIt. 83 N.2
(in press
The ATLAS-SPT Radio Survey of Cluster Galaxies
Using a high-performance computing cluster to mosaic 4,787 pointings, we have
imaged the 100 sq. deg. South Pole Telescope (SPT) deep-field at 2.1 GHz using
the Australian Telescope Compact Array to an rms of 80 Jy and a resolution
of 8". Our goal is to generate an independent sample of radio-selected galaxy
clusters to study how the radio properties compare with cluster properties at
other wavelengths, over a wide range of redshifts in order to construct a
timeline of their evolution out to . A preliminary analysis of the
source catalogue suggests there is no spatial correlation between the clusters
identified in the SPT-SZ catalogue and our wide-angle tail galaxies.Comment: 9 pages, 4 figures. Submitted to Proceedings of Science for "The many
facets of extragalactic radio surveys: towards new scientific challenges",
Bologna, Italy 20-23 October 2015 (EXTRA-RADSUR2015
Radio Frequency Models of Novae in eruption. I. The Free-Free Process in Bipolar Morphologies
Observations of novae at radio frequencies provide us with a measure of the
total ejected mass, density profile and kinetic energy of a nova eruption. The
radio emission is typically well characterized by the free-free emission
process. Most models to date have assumed spherical symmetry for the eruption,
although it has been known for as long as there have been radio observations of
these systems, that spherical eruptions are to simplistic a geometry. In this
paper, we build bipolar models of the nova eruption, assuming the free-free
process, and show the effects of varying different parameters on the radio
light curves. The parameters considered include the ratio of the minor- to
major-axis, the inclination angle and shell thickness (further parameters are
provided in the appendix). We also show the uncertainty introduced when fitting
spherical model synthetic light curves to bipolar model synthetic light curves.
We find that the optically thick phase rises with the same power law () for both the spherical and bipolar models. In the bipolar case
there is a "plateau" phase -- depending on the thickness of the shell as well
as the ratio of the minor- to major-axis -- before the final decline, that
follows the same power law () as in the spherical case.
Finally, fitting spherical models to the bipolar model synthetic light curves
requires, in the worst case scenario, doubling the ejected mass, more than
halving the electron temperature and reducing the shell thickness by nearly a
factor of 10. This implies that in some systems we have been over predicting
the ejected masses and under predicting the electron temperature of the ejecta.Comment: 9 pages, 6 figures, accepted for publication in ApJ, accompanying
movie to figure 3 available at
http://www.ast.uct.ac.za/~valerio/papers/radioI
The Market for Teacher Quality
Much of education policy focuses on improving teacher quality, but most policies lack strong research support. We use student achievement gains to estimate teacher value-added, our measure of teacher quality. The analysis reveals substantial variation in the quality of instruction, most of which occurs within rather than between schools. Although teacher quality appears to be unrelated to advanced degrees or certification, experience does matter -- but only in the first year of teaching. We also find that good teachers tend to be effective with all student ability levels but that there is a positive value of matching students and teachers by race. In the second part of the analysis, we show that teachers staying in our sample of urban schools tend to be as good as or better than those who exit. Thus, the main cost of large turnover is the introduction of more first year teachers. Finally, there is little or no evidence that districts that offer higher salaries and have better working conditions attract the higher quality teachers among those who depart the central city district. The overall results have a variety of direct policy implications for the design of school accountability and the compensation of teachers.
Swift observations of the 2006 outburst of the recurrent nova RS Ophiuchi: III. X-ray spectral modelling
Following the Swift X-ray observations of the 2006 outburst of the recurrent
nova RS Ophiuchi, we developed hydrodynamical models of mass ejection from
which the forward shock velocities were used to estimate the ejecta mass and
velocity. In order to further constrain our model parameters, here we present
synthetic X-ray spectra from our hydrodynamical calculations which we compare
to the Swift data. An extensive set of simulations was carried out to find a
model which best fits the spectra up to 100 days after outburst. We find a good
fit at high energies but require additional absorption to match the low energy
emission. We estimate the ejecta mass to be in the range (2-5) x 10^{-7} solar
masses and the ejection velocity to be greater than 6000 km/s (and probably
closer to 10,000 km/s). We also find that estimates of shock velocity derived
from gas temperatures via standard model fits to the X-ray spectra are much
lower than the true shock velocities.Comment: 13 pages, 5 figures, Accepted for publication in Ap
Gravitational Collapse of Self-Similar and Shear-free Fluid with Heat Flow
A class of solutions to Einstein field equations is studied, which represents
gravitational collapse of thick spherical shells made of self-similar and
shear-free fluid with heat flow. It is shown that such shells satisfy all the
energy conditions, and the corresponding collapse always forms naked
singularities.Comment: 34 pages, 9 figures, late
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