86,604 research outputs found
Modelling repeated epidemics with general infection kernels
An integral equation approach is taken to explore the characteristics of
a general infectious disease in a homogeneous population. It is shown that
the final size of the epidemic depends on the basic reproduction ratio for
the infection and the initial number of susceptibles. A discrete map for the
susceptible population from epidemic generation to epidemic generation is
formed to consider the long term behaviour of the disease in a population of
constant size
Relaxational dislocation damping due to dislocation-dislocation intersections with application to magnesium single crystals
Relaxational dislocation damping due to dislocation-dislocation intersections with applications to magnesium single crystal
Analysis of the gain distribution across the active region of InGaAs-InAlGaAs multiple quantum well lasers
Spectral gain measurements for two InGaAs-InAlGaAs multiple width quantum well structures, with inverse-configured active regions, have been presented. One structure consisted of wide quantum wells near the p-side and narrow quantum wells near the n-side of the active region. The other structure consisted of narrow quantum wells near the p-side of the active region with wider quantum wells near the n-side. It is shown that, for the same operating conditions, the structure with wide quantum wells on the p-side of the active region provided a 15% broader gain spectrum in comparison to the structure with narrow quantum wells on the p-side of the active region. The analysis of the results shows non-uniform carrier distribution across the active region of the structures, where the structure with wide quantum wells near the p-side of the active region provided 65% more gain in comparison to the structure with narrow quantum wells near the p-side of the active region. The gain distribution results have been compared with that obtained for the phosphorous quaternary structures in other literature and have shown there is some evidence to suggest that the gain distribution is more uniform in aluminium quaternary than phosphorous quaternary material
A model for evolution and extinction
We present a model for evolution and extinction in large ecosystems. The
model incorporates the effects of interactions between species and the
influences of abiotic environmental factors. We study the properties of the
model by approximate analytic solution and also by numerical simulation, and
use it to make predictions about the distribution of extinctions and species
lifetimes that we would expect to see in real ecosystems. It should be possible
to test these predictions against the fossil record. The model indicates that a
possible mechanism for mass extinction is the coincidence of a large
coevolutionary avalanche in the ecosystem with a severe environmental
disturbance.Comment: Postscript (compressed etc. using uufiles), 16 pages, with 15
embedded figure
Estimating a preference-based index for a menopause specific health quality of life questionnaire
BACKGROUND: The aim of the study was to develop a menopause-specific, preference-based healthrelated
quality-of-life (HRQoL) index reflecting both menopausal symptoms and potential sideeffects
of Hormone Replacement Therapy (HRT).
METHODS: The study had three phases: the development of a health state classification, a
prospective valuation survey and the estimation of a model to interpolate HRQoL indices for all
remaining health states as defined by the classification. A menopausal health state classification was
developed with seven dimensions: hot flushes, aching joints/muscles, anxious/frightened feelings,
breast tenderness, bleeding, vaginal dryness and undesirable androgenic signs. Each dimension
contains between three and five levels and defines a total of 6,075 health states. A sample of 96
health states was selected for the valuation survey. These states were valued by a sample of 229
women aged 45 to 60, randomly selected from 6 general practice lists in Sheffield, UK. Respondents
were asked to complete a time trade-off (TTO) task for nine health states, resulting in an average
of 16.5 values for each health state.
RESULTS: Mean health states valued range from 0.48 to 0.98 (where 1.0 is full health and zero is for
states regarded as equivalent to death). Symptoms, as described by the classification system, can be
rank-ordered in terms of their impact (from high to low) on menopausal HRQoL as follows: aching
joints and muscles, bleeding, breast tenderness, anxious or frightened feelings, vaginal dryness,
androgenic signs. Hot flushes did not significantly contribute to model fit. The preferred model
produced a mean absolute error of 0.053, but suffered from bias at both ends of the scale.
CONCLUSION: This article presents an attempt to directly value a condition specific health state
classification. The overall fit was disappointing, but the results demonstrate that menopausal symptoms are perceived by patients to have a significant impact on utility. The overall effect is modest compared to the more generic health state descriptions such as the EQ-5D. The resultant
algorithm generates a preference-based index that can be used economic evaluation and that reflects the impact of this condition
- âŠ