299 research outputs found
The vulnerability of rules in complex work environments: dynamism and uncertainty pose problems for cognition
Many complex work environments rely heavily on cognitive operators using rules. Operators sometimes fail to implement rules, with catastrophic human, social and economic costs. Rule-based error is widely reported, yet the mechanisms of rule vulnerability have received less attention. This paper examines rule vulnerability in the complex setting of airline transport operations. We examined ‘the stable approach criteria rule’, which acts as a system defence during the approach to land. The study experimentally tested whether system state complexity influenced rule failure. The results showed increased uncertainty and dynamism led to increased likelihood of rule failure. There was also an interaction effect, indicating complexity from different sources can combine to further constrain rule-based response. We discuss the results in relation to recent aircraft accidents and suggest that ‘rule-based error’ could be progressed to embrace rule vulnerability, fragility and failure. This better reflects the influence that system behaviour and cognitive variety have on rule-based response. Practitioner Summary: In this study, we examined mechanisms of rule vulnerability in the complex setting of airline transport operations. The results suggest work scenarios featuring high uncertainty and dynamism constrain rule-based response, leading to rules becoming vulnerable, fragile or failing completely. This has significant implications for rule-intensive, safety critical work environments
A qualitative optimization technique for biophysical neuron models with many parameters
We present a novel computational technique that enables more efficient optimization of qualitative features in biophysical neural models
Geometric analysis of soft thresholds in action potential initiation and the consequences for understanding phase response curves and model tuning
Distant field BHB stars and the mass of the Galaxy II: Photometry and spectroscopy of UKST candidates 16<B<19.5, 11<R<52 kpc
This is the second in a series of papers presenting a new calculation of the
mass of the Galaxy based on radial velocities and distances for a sample of
faint 16 < B < 21.3 field blue horizontal-branch (BHB) stars. We present
accurate BV CCD photometry and spectra for 142 candidate A-type stars selected
from ub_jr photometry of UK Schmidt telescope plates in six
high-Galactic-latitude fields. Classification of these candidates produces a
sample of 60 BHB stars at distances of 11-52 kpc from the Sun (mean 28 kpc),
with heliocentric line-of-sight velocities accurate to 15 km/s, and distance
errors < 10%. We provide a summary table listing coordinates and velocities of
these stars. The measured dispersion of the radial component of the
Galactocentric velocity for this sample is 108+-10 km/s, in agreement with a
recent study of the distant halo by Sirko and coworkers. Measurements of the Ca
II K line indicate that nearly all the stars are metal-poor with a mean [Fe/H]
= -1.8 with dispersion 0.5. Subsequent papers will describe a second survey of
BHBs to heliocentric distances 70 < R < 125 kpc and present a new estimate of
the mass of the Galaxy.Comment: 16 pages, 15 figures. Accepted for publication in MNRA
Evidence of a link between the evolution of clusters and their AGN fraction
We discuss the optical properties, X-ray detections and active galactic nucleus (AGN) populations of four clusters at z ~ 1 in the Subaru–XMM Deep Field (SXDF). The velocity distribution and plausible extended X-ray detections are examined, as well as the number of X-ray point sources and radio sources associated with the clusters. We find that the two clusters that appear virialized and have an extended X-ray detection contain few, if any, AGN, whereas the two pre-virialized clusters have a large AGN population. This constitutes evidence that the AGN fraction in clusters is linked to the clusters' evolutionary stage. The number of X-ray AGN in the pre-virialized clusters is consistent with an overdensity of a factor of ~200; the radio AGN appear to be clustered with a factor of 3 to 6 higher. The median K-band luminosities of L_K= 1.7 ± 0.7 L* for the X-ray sources and L_K= 2.3 ± 0.1 L* for the radio sources support the theory that these AGN are triggered by galaxy interaction and merging events in sub-groups with low internal velocity distributions, which make up the cluster environment in a pre-virialization evolutionary stage
Timing and Reconstruction of the Most Recent Common Ancestor of the Subtype C Clade of Human Immunodeficiency Virus Type 1
Human immunodeficiency virus type 1 (HIV-1) subtype C is responsible for more than 55% of HIV-1 infections worldwide. When this subtype first emerged is unknown. We have analyzed all available gag (p17 and p24) and env (C2-V3) subtype C sequences with known sampling dates, which ranged from 1983 to 2000. The majority of these sequences come from the Karonga District in Malawi and include some of the earliest known subtype C sequences. Linear regression analyses of sequence divergence estimates (with four different approaches)were plotted against sample year to estimate the year in which there was zero divergence from the reconstructed ancestral sequence. Here we suggest that the most recent common ancestor of subtype C
appeared in the mid- to late 1960s. Sensitivity analyses, by which possible biases due to oversampling from one
district were explored, gave very similar estimates
Distant Field BHB Stars III: Identification of a probable outer halo stream at Galactocentric distance r = 70 kpc
We present VLT-FORS1 spectra of a sample of 34 faint 20.0 < g* < 21.1 A-type
stars selected from the Sloan Digital Sky Survey Early Data Release, with the
goal of measuring the velocity dispersion of blue horizontal branch (BHB) stars
in the remote Galactic halo, R~80kpc. We show that colour selection with 1.08 <
u*-g* < 1.40 and -0.2 < g*-r* < -0.04 minimises contamination of the sample by
less luminous blue stragglers. In classifying the stars we confine our
attention to the 20 stars with spectra of signal-to-noise ratio > 15 per
Angstrom. Classification produces a sample of eight BHB stars at distances
65-102 kpc from the Sun (mean 80 kpc), which represents the most distant sample
of Galactic stars with measured radial velocities. The dispersion of the
measured radial component of the velocity with respect to the centre of the
Galaxy is 58+-15km/s. This value is anomalously low in comparison with measured
values for stars at smaller distances, as well as for satellites at similar
distances. Seeking an explanation for the low measured velocity dispersion,
further analysis reveals that six of the eight remote BHB stars are plausibly
associated with a single orbit. Three previously known outer halo carbon stars
also appear to belong to this stream. The velocity dispersion of all nine stars
relative to the orbit is only 15+-4 km/s. Further observations along the orbit
are required to trace the full extent of this structure on the sky.Comment: 13 pages, 7 figures. Accepted for publication in MNRA
Reconciling the contribution of environmental and stochastic structuring of tropical forest diversity through the lens of imaging spectroscopy.
Both niche and stochastic dispersal processes structure the extraordinary diversity of tropical plants, but determining their relative contributions has proven challenging. We address this question using airborne imaging spectroscopy to estimate canopy β-diversity for an extensive region of a Bornean rainforest and challenge these data with models incorporating niches and dispersal. We show that remotely sensed and field-derived estimates of pairwise dissimilarity in community composition are closely matched, proving the applicability of imaging spectroscopy to provide β-diversity data for entire landscapes of over 1000 ha containing contrasting forest types. Our model reproduces the empirical data well and shows that the ecological processes maintaining tropical forest diversity are scale dependent. Patterns of β-diversity are shaped by stochastic dispersal processes acting locally whilst environmental processes act over a wider range of scales
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