5,376 research outputs found
Computationally efficient methods for fitting mixed models to electronic health records data
Motivated by two case studies using primary care records from the Clinical Practice Research Datalink, we describe statistical methods that facilitate the analysis of tall data, with very large numbers of observations. Our focus is on investigating the association between patient characteristics and an outcome of interest, while allowing for variation among general practices. We explore ways to fit mixed effects models to tall data, including predictors of interest and confounding factors as covariates, and including random intercepts to allow for heterogeneity in outcome among practices. We introduce: (1) weighted regression and (2) meta-analysis of estimated regression coefficients from each practice. Both methods reduce the size of the dataset, thus decreasing the time required for statistical analysis. We compare the methods to an existing subsampling approach. All methods give similar point estimates, and weighted regression and meta-analysis give similar standard errors for point estimates to analysis of the entire dataset, but the subsampling method gives larger standard errors. Where all data are discrete, weighted regression is equivalent to fitting the mixed model to the entire dataset. In the presence of a continuous covariate, meta-analysis is useful. Both methods are easy to implement in standard statistical softwareThe authors are grateful to the CPRD team at the University of Cambridge. In particular, we thank Carol Wilson and Anna Cassel for providing access to the case study datasets that they spent much time preparing for analysis. Kirsty Rhodes was funded by Medical Research Council Unit Programmes U105260558 and MC_UU_00002/5. Rebecca Turner and Ian White were funded by Medical Research Council Unit Programmes U105260558 and MC_UU_12023/2
Emission Line Galaxies in the STIS Parallel Survey II: Star Formation Density
We present the luminosity function of [OII]-emitting galaxies at a median
redshift of z=0.9, as measured in the deep spectroscopic data in the STIS
Parallel Survey (SPS). The luminosity function shows strong evolution from the
local value, as expected. By using random lines of sight, the SPS measurement
complements previous deep single field studies. We calculate the density of
inferred star formation at this redshift by converting from [OII] to H-alpha
line flux as a function of absolute magnitude and find rho_dot=0.043 +/- 0.014
Msun/yr/Mpc^3 at a median redshift z~0.9 within the range 0.46<z<1.415 (H_0 =
70 km/s/Mpc, Omega_M=0.3, Omega_Lambda=0.7. This density is consistent with a
(1+z)^4 evolution in global star formation since z~1. To reconcile the density
with similar measurements made by surveys targeting H-alpha may require
substantial extinction correction.Comment: 16 preprint pages including 5 figures; accepted for publication in
Ap
Tackling concentrated worklessness: integrating governance and policy across and within spatial scales
Spatial concentrations of worklessness remained a key characteristic of labour markets in advanced industrial economies, even during the period of decline in aggregate levels of unemployment and economic inactivity evident from the late 1990s to the economic downturn in 2008. The failure of certain localities to benefit from wider improvements in regional and national labour markets points to a lack of effectiveness in adopted policy approaches, not least in relation to the governance arrangements and policy delivery mechanisms that seek to integrate residents of deprived areas into wider local labour markets. Through analysis of practice in the British context, we explore the difficulties of integrating economic and social policy agendas within and across spatial scales to tackle problems of concentrated worklessness. We present analysis of a number of selected case studies aimed at reducing localised worklessness and identify the possibilities and constraints for effective action given existing governance arrangements and policy priorities to promote economic competitiveness and inclusion
Gaussian multiplicative Chaos for symmetric isotropic matrices
Motivated by isotropic fully developed turbulence, we define a theory of
symmetric matrix valued isotropic Gaussian multiplicative chaos. Our
construction extends the scalar theory developed by J.P. Kahane in 1985
Lognormal scale invariant random measures
In this article, we consider the continuous analog of the celebrated
Mandelbrot star equation with lognormal weights. Mandelbrot introduced this
equation to characterize the law of multiplicative cascades. We show existence
and uniqueness of measures satisfying the aforementioned continuous equation;
these measures fall under the scope of the Gaussian multiplicative chaos theory
developed by J.P. Kahane in 1985 (or possibly extensions of this theory). As a
by product, we also obtain an explicit characterization of the covariance
structure of these measures. We also prove that qualitative properties such as
long-range independence or isotropy can be read off the equation.Comment: 31 pages; Probability Theory and Related Fields (2012) electronic
versio
Another derivation of the geometrical KPZ relations
We give a physicist's derivation of the geometrical (in the spirit of
Duplantier-Sheffield) KPZ relations, via heat kernel methods. It gives a
covariant way to define neighborhoods of fractals in 2d quantum gravity, and
shows that these relations are in the realm of conformal field theory
Emission Line Galaxies in the STIS Parallel Survey I: Observations and Data Analysis
In the first three years of operation STIS obtained slitless spectra of
approximately 2500 fields in parallel to prime HST observations as part of the
STIS Parallel Survey (SPS). The archive contains almost 300 fields at high
galactic latitude (|b|>30) with spectroscopic exposure times greater than 3000
seconds. This sample contains 220 fields (excluding special regions and
requiring a consistent grating angle) observed between 6 June 1997 and 21
September 2000, with a total survey area of about 160 square arcminutes. At
this depth, the SPS detects an average of one emission line galaxy per three
fields. We present the analysis of these data, and the identification of 131
low to intermediate redshift galaxies detected by optical emission lines. The
sample contains 78 objects with emission lines that we infer to be redshifted
[OII]3727 emission at 0.43<z<1.7. The comoving number density of these objects
is comparable to that of H-alpha emitting galaxies in the NICMOS parallel
observations. One quasar and three probable Seyfert galaxies are detected. Many
of the emission-line objects show morphologies suggestive of mergers or
interactions. The reduced data are available upon request from the authors.Comment: 58 preprint pages, including 26 figures; accepted for publication in
ApJ
Characterizing Invasiveness through a Descriptive Study of Guinea Grass (\u3ci\u3eMegathyrsus maximus\u3c/i\u3e) Growing in Three Habitat Types and Differing Herbivore Assemblages in Both Kenya and Texas
Guinea grass (Megathyrsus maximus syn. Panicum maximum) is an important pasture grass that has been introduced pantropically, yet in many cases has escaped cultivation and is invading native rangelands – threatening biodiversity at multiple trophic levels. An increasing challenge of rangeland management is balancing the importance of pasture grasses with their negative impact on ecosystem processes. Given this challenge, it is critical to understand the mechanisms that underlie grass invasion. In this descriptive study, our objective was to assess the ecological stress release hypothesis by comparing the relative abundance and functional traits of Guinea grass in Kenya\u27s home range with its introduced range in Texas, USA. To characterize Guinea grass dominance in Texas vs. Kenya, we measured plant height and cover and examined the associated arthropod and ungulate abundances. We characterized these associations across three habitat types of differing resource availability; 1) Under woody legumes with higher soil moisture and soil nitrogen, 2) grasslands with higher light yet drier soils, 3) riparian areas with higher soil water moisture and light. We found that Texas Guinea grass was 50% taller than Kenyan Guinea grass except for riparian environments comparable to its introduced range. Texan Guinea grass occupied 50% more area than Kenyan Guinea grass and was especially abundant under woody legumes. Texas ungulate communities were less diverse and abundant. The arthropod herbivore diversity was twice as high on Kenyan Guinea grass. These results suggest that Guinea grass has escaped a rich assemblage of herbivores and likely explains some of its spread in nonnative ranges. The interaction between resource environment and herbivory may elucidate mechanisms important for Guinea grass escaping cultivation in its introduced range and lays the foundation for future ecological studies and a search for biological control
Does elite European match-play affect salivary immunoglobulin- a and cortisol in soccer players? The influence of playing status and match outcome
Introduction: The aims of this study were to: a) investigate salivary
immunoglobulin A (s-IgA) and cortisol (s-Cort) responses to nine competitive
fixtures in starting and non- starting soccer players; and b) compare s-IgA and s Cort responses of starters and non-starters considering match outcome.
Methods: Saliva from 19 male outfield players from an elite soccer team (mean ±
SD, age 26 ± 4 years; weight 80.5 ± 8.1 kg; height 1.83 ± 0.07 m; body-fat 10.8% ±
0.7%) was collected. Saliva samples were taken on the day before each match
(MD-1), 60-min before kick-off (MDpre), 30-min post-match (MDpost), and 72-h
post-match (MD+3). There were five wins, one draw and three losses.
Results: The mean s-IgA value was found to be significantly lower at MD+3
compared to MDpre and MDpost. s-Cort was significantly higher at MDpost
compared to MD-1 and MDpre. When compared to MDpre, a statistically
significant decrease in s-Cort was observed at MD+3 compared to MDpost.
Starters displayed higher s-Cort values across the nine matches. There was a
significant group-by-time interaction for s-Cort. There was a significant increase
in s-Cort levels at MDpost compared to MD-1 and from MDpre to MDpost in
starting players. At MDpost, starters had significantly higher s-Cort values. s-IgA
values of starting and non- starting players following successful and unsuccessful
matches did not reveal a significant difference. However, similar analysis of s-Cort
in successful matches showed a significant difference between starters and non starters. s-IgA values at MD-1, MDpre, MDpost and MD+3 in starters and non starters following successful and unsuccessful matches revealed significant
differences at MDpre and MDpost in starters, respectively. Furthermore, s-Cort
values at MD-1, MDpre, MDpost and MD+3 in starters and non-starters in
successful and unsuccessful matches revealed significant differences at MD+3
in starting players.Discussion: The present study suggests that in elite level soccer players, both
starting status and match outcome influence s-IgA and s-Cort responses,
particularly starters. Specifically, s-IgA was lower for starters before and after
the match following successful outcomes. Moreover, higher s-Cort values were
found before the match while lower values occurred after the match for starters in
successful matches.info:eu-repo/semantics/publishedVersio
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