930 research outputs found
Development of a New Word List
Recent work in the field of criterion-referenced measurement has emphasized the key communicative role played by a test\u27s specifications, that is, the rules employed to generate the actual items used on the test. Sometimes referred to as domain specifications, II since those specifications make operational the domain of behaviors being assessed by the test, the specifications provide teachers with the skill definitions needed to organize their instruction. The more lucid such specifications are, the more likely educators will understand the skill being sought, and the more likely that they will design appropriate instructional sequences
OBTAINING MODELS FOR ALFALFA, SORGHUM, AND WHEAT RESIDUE DECOMPOSITION
Crop residues provide an economical means for controlling wind and water erosion in addition to being a valuable source of plant nutrients. As residues decompose they lose nutrients, mass and the ability to protect the soil surface from erosive forces. The research was designed to evaluate rates of residue decomposition of sorghum, wheat and alfalfa on the soil surface and buried, in five soil moisture regimes. Moisture was applied to soil by line source irrigation and bags containing crop residues were retrieved and analyzed across time. Thus, observations were repeated in both space and time .
Wieder and Lang (1982) reported that mass-loss over time was modeled well by the negative exponential. Because residue can be divided into fast (labile) and slow (recalcitrant) decomposing fractions, the double exponential is suggested. Assuming the ratio of labile to recalcitrant is constant for. a crop regardless of soil moisture, and whether on the surface or buried, it would be sufficient for each crop to fit a set of simultaneous non-linear functions with three parameters, a constant A (proportion labile) over all equations with different k1,\u27s (labile fraction decomposition rates) and k 2\u27s (recalcitrant decomposition rates) for soil moisture levels and whether buried or unburied .
For alfalfa the results were consistent with the above theory. For wheat and sorghum data holding A constant over all environments resulted in k\u27 s \u3e 0. Convergence of the estimations process could not be obtained when forcing k\u27s ≤ 0. The single exponential provided a satisfactory model of decomposition, but without the advantage of separating the residues into labile and recalcitrant fractions. The inability to obtain estimates using the double exponential apparently resulted from an insufficient observation period. The recalcitrant fraction of the surface residues of these crops had not disappeared after more than a year
Analysis of the Spore Membrane Proteome in Clostridium perfringens Implicates Cyanophycin in Spore Assembly.
UnlabelledHeat-resistant endospore formation plays an important role in Clostridium perfringens-associated foodborne illnesses. The spores allow the bacterium to survive heating during normal cooking processes, followed by germination and outgrowth of the bacterium in contaminated foods. To identify proteins associated with germination and other spore functions, a comparative spore membrane proteome analysis of dormant and germinated spores of C. perfringens strain SM101 was performed by using gel-based protein separation and liquid chromatography coupled with matrix-assisted laser desorption ionization-tandem time of flight (MALDI-TOF/TOF) mass spectrometry. A total of 494 proteins were identified, and 117 of them were predicted to be integral membrane or membrane-associated proteins. Among these membrane proteins, 16 and 26 were detected only in dormant and germinated spores, respectively. One protein that was detected only in germinated spore membranes was the enzyme cyanophycinase, a protease that cleaves the polymer cyanophycin, which is composed of l-arginine-poly(l-aspartic acid), to β-Asp-Arg. Genes encoding cyanophycinase and cyanophycin synthetase have been observed in many species of Clostridium, but their role has not been defined. To determine the function of cyanophycin in C. perfringens, a mutation was introduced into the cphA gene, encoding cyanophycin synthetase. In comparison to parent strain SM101, the spores of the mutant strain retained wild-type levels of heat resistance, but fewer spores were made, and they were smaller, suggesting that cyanophycin synthesis plays a role in spore assembly. Although cyanophycin could not be extracted from sporulating C. perfringens cells, an Escherichia coli strain expressing the cphA gene made copious amounts of cyanophycin, confirming that cphA encodes a cyanophycin synthetase.ImportanceClostridium perfringens is a common cause of food poisoning, and germination of spores after cooking is thought to play a significant role in the disease. How C. perfringens controls the germination process is still not completely understood. We characterized the proteome of the membranes from dormant and germinated spores and discovered that large-scale changes occur after germination is initiated. One of the proteins that was detected after germination was the enzyme cyanophycinase, which degrades the storage compound cyanophycin, which is found in cyanobacteria and other prokaryotes. A cyanophycin synthetase mutant was constructed and found to make spores with altered morphology but normal heat resistance, suggesting that cyanophycin plays a different role in C. perfringens than it does in cyanobacteria
Causal Viscosity in Accretion Disc Boundary Layers
The structure of the boundary layer region between the disc and a
comparatively slowly rotating star is studied using a causal prescription for
viscosity. The vertically integrated viscous stress relaxes towards its
equilibrium value on a relaxation timescale , which naturally yields a
finite speed of propagation for viscous information. For a standard alpha
prescription with alpha in the range 0.1-0.01, and ratio of viscous speed to
sound speed in the range 0.02-0.5, details in the boundary layer are strongly
affected by the causality constraint. We study both steady state polytropic
models and time dependent models, taking into account energy dissipation and
transport. Steady state solutions are always subviscous with a variety of
profiles which may exhibit near discontinuities. For alpha =0.01 and
small viscous speeds, the boundary layer adjusted to a near steady state. A
long wavelength oscillation generated by viscous overstability could be seen at
times near the outer boundary. Being confined there, the boundary layer
remained almost stationary. However, for alpha =0.1 and large viscous speeds,
short wavelength disturbances were seen throughout which could significantly
affect the power output in the boundary layer. This could be potentially
important in producing time dependent behaviour in accreting systems such as
CVs and protostars.Comment: 10 LateX pages, requires lamuphys.sty and psfig.sty, 3 figures
included, to appear in the Proceedings of the EARA Workshop on Accretion
Disks (Garching, Oct. 96), Lecture Notes in Physic
Raising standards in American schools: the case of No Child Left Behind
In January 2002, President George W Bush signed into law what is arguably the most important piece of US educational legislation for the past 35 years. For the first time, Public Law 107-110 links high stakes testing with strict accountability measures designed to ensure that, at least in schools that receive government funding, no child is left behind. The appropriately named No Child Left Behind Act (NCLB) links government funding to strict improvement policies for America’s public schools. Much of what is undertaken in NCLB is praiseworthy, the Act is essentially equitable for it ensures that schools pay due regard to the progress of those sections of the school population who have traditionally done less well in school, in particular, students from economically disadvantaged homes, as well as those from ethnic minority backgrounds and those who have limited proficiency to speak English. However, this seemingly salutatory aspect of the Act is also the one that has raised the most objections. This paper describes the key features of this important piece of legislation before outlining why it is that a seemingly equitable Act has produced so much consternation in US education circles. Through an exploration of school level data for the state of New Jersey, the paper considers the extent to which these concerns have been justified during the early days of No Child Left Behind
Hyper-Accreting Black Holes and Gamma-Ray Bursts
A variety of current models for gamma-ray bursts (GRBs) suggest a common
engine - a black hole of several solar masses accreting matter from a disk at a
rate 0.01 to 10 solar masses per second. Using a numerical model for
relativistic disk accretion, we have studied steady-state accretion at these
high rates. Inside a radius ~ 10**8 cm, for accretion rates greater than about
0.01 solar masses per second, a global state of balanced power comes to exist
between neutrino losses, chiefly pair capture on nucleons, and dissipation.
Energy emitted in neutrinos is less, and in the case of low accretion rates,
very much less, than the maximum efficiency factor for black hole accretion
(0.057 for no rotation; 0.42 for extreme Kerr rotation) times Mdot c**2. The
efficiency for producing a pair fireball along the rotational axis by neutrino
annihilation is calculated and found to be highly variable and very sensitive
to the accretion rate. For some of the higher accretion rates studied, it can
be several per cent or more; for accretion rates less than 0.05 solar masses
per second, it is essentially zero. The efficiency of the Blandford-Znajek
mechanism in extracting rotational energy from the black hole is also
estimated. In light of these results, the viability of various gamma-ray burst
models is discussed and the sensitivity of the results to disk viscosity, black
hole rotation rate, and black hole mass explored. A diverse range of GRB
energies seems unavoidable and neutrino annihilation in hyper-accreting black
hole systems can explain bursts up to 10**52 erg. Larger energies may be
inferred for beaming systems.Comment: 46 pages, includes 9 figures, LaTeX (uses aaspp4.sty), accepted by
The Astrophysical Journal. Additional solutions in Tables and Figs. 4 and 5,
minor revisions to text, references adde
Revisiting vertical structure of neutrino-dominated accretion disks: Bernoulli parameter, neutrino trapping and other distributions
We revisit the vertical structure of neutrino dominated accretion flows
(NDAFs) in spherical coordinates with a new boundary condition based on the
mechanical equilibrium. The solutions show that NDAF is significantly thick.
The Bernoulli parameter and neutrino trapping are determined by the mass
accretion rate and the viscosity parameter. According to the distribution of
the Bernoulli parameter, the possible outflow may appear in the outer region of
the disk. The neutrino trapping can essentially affect the neutrino radiation
luminosity. The vertical structure of NDAF is like a "sandwich", and the
multilayer accretion may account for the flares in gamma-ray bursts.Comment: 7 pages, 2 figures, Accepted for publication in Astrophysics & Space
Scienc
Jet Precession Driven by Neutrino-Cooled Disc for Gamma-Ray Bursts
A model of jet precession driven by a neutrino-cooled disc around a spinning
black hole is present in order to explain the temporal structure and spectral
evolution of gamma-ray bursts (GRBs). The differential rotation of the outer
part of a neutrino dominated accretion disc may result in precession of the
inner part of the disc and the central black hole, hence drives a precessed jet
via neutrino annihilation around the inner part of the disc. Both analytic and
numeric results for our model are present. Our calculations show that a black
hole-accretion disk system with black hole mass ,
accretion rate , spin parameter
and viscosity parameter may drive a precessed jet with period P=1
s and luminosity erg s, corresponding to the scenario for
long GRBs. A precessed jet with s and erg s may be
powered by a system with , , , and , possibly being responsible for the
short GRBs. Both the temporal and spectral evolution in GRB pulse may explained
with our model. GRB central engines likely power a precessed jet driven by a
neutrino-cooled disc. The global GRB lightcurves thus could be modulated by the
jet precession during the accretion timescale of the GRB central engine. Both
the temporal and spectral evolution in GRB pulse may be due to an viewing
effect due to the jet precession.Comment: 5 pages, 4 figures, accepted for publication in Astronomy and
Astrophysic
Advection-Dominated Accretion Model of Sagittarius A*: Evidence for a Black Hole at the Galactic Center
Sgr A* at the Galactic Center is a puzzling source. It has a mass
M=(2.5+/-0.4) x 10^6 solar masses which makes it an excellent black hole
candidate. Observations of stellar winds and other gas flows in its vicinity
suggest a mass accretion rate approximately few x 10^{-6} solar masses per
year. However, such an accretion rate would imply a luminosity > 10^{40} erg/s
if the radiative efficiency is the usual 10 percent, whereas observations
indicate a bolometric luminosity <10^{37} erg/s. The spectrum of Sgr A* is
unusual, with emission extending over many decades of wavelength. We present a
model of Sgr A* which is based on a two-temperature optically-thin
advection-dominated accretion flow. The model is consistent with the estimated
mass and accretion rate, and fits the observed fluxes in the cm/mm and X-ray
bands as well as upper limits in the sub-mm and infrared bands; the fit is less
good in the radio below 86 GHz and in gamma-rays above 100 MeV. The very low
luminosity of Sgr A* is explained naturally in the model by means of advection.
Most of the viscously dissipated energy is advected into the central mass by
the accreting gas, and therefore the radiative efficiency is extremely low,
approximately 5 x 10^{-6}. A critical element of the model is the presence of
an event horizon at the center which swallows the advected energy. The success
of the model could thus be viewed as confirmation that Sgr A* is a black hole.Comment: 41 pages (Latex) including 6 Figures and 2 Tables. Final Revised
Version changes to text, tables and figures. ApJ, 492, in pres
Using item response theory to explore the psychometric properties of extended matching questions examination in undergraduate medical education
BACKGROUND:
As assessment has been shown to direct learning, it is critical that the examinations developed to test clinical competence in medical undergraduates are valid and reliable. The use of extended matching questions (EMQ) has been advocated to overcome some of the criticisms of using multiple-choice questions to test factual and applied knowledge.
METHODS:
We analysed the results from the Extended Matching Questions Examination taken by 4th year undergraduate medical students in the academic year 2001 to 2002. Rasch analysis was used to examine whether the set of questions used in the examination mapped on to a unidimensional scale, the degree of difficulty of questions within and between the various medical and surgical specialties and the pattern of responses within individual questions to assess the impact of the distractor options.
RESULTS:
Analysis of a subset of items and of the full examination demonstrated internal construct validity and the absence of bias on the majority of questions. Three main patterns of response selection were identified.
CONCLUSION:
Modern psychometric methods based upon the work of Rasch provide a useful approach to the calibration and analysis of EMQ undergraduate medical assessments. The approach allows for a formal test of the unidimensionality of the questions and thus the validity of the summed score. Given the metric calibration which follows fit to the model, it also allows for the establishment of items banks to facilitate continuity and equity in exam standards
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