3,871 research outputs found
Biased-estimations of the Variance and Skewness
Nonlinear combinations of direct observables are often used to estimate
quantities of theoretical interest. Without sufficient caution, this could lead
to biased estimations. An example of great interest is the skewness of
the galaxy distribution, defined as the ratio of the third moment \xibar_3
and the variance squared \xibar_2^2. Suppose one is given unbiased estimators
for \xibar_3 and \xibar_2^2 respectively, taking a ratio of the two does
not necessarily result in an unbiased estimator of . Exactly such an
estimation-bias affects most existing measurements of . Furthermore,
common estimators for \xibar_3 and \xibar_2 suffer also from this kind of
estimation-bias themselves: for \xibar_2, it is equivalent to what is
commonly known as the integral constraint. We present a unifying treatment
allowing all these estimation-biases to be calculated analytically. They are in
general negative, and decrease in significance as the survey volume increases,
for a given smoothing scale. We present a re-analysis of some existing
measurements of the variance and skewness and show that most of the well-known
systematic discrepancies between surveys with similar selection criteria, but
different sizes, can be attributed to the volume-dependent estimation-biases.
This affects the inference of the galaxy-bias(es) from these surveys. Our
methodology can be adapted to measurements of analogous quantities in quasar
spectra and weak-lensing maps. We suggest methods to reduce the above
estimation-biases, and point out other examples in LSS studies which might
suffer from the same type of a nonlinear-estimation-bias.Comment: 28 pages of text, 9 ps figures, submitted to Ap
Non-Markovian Decay of a Three Level Cascade Atom in a Structured Reservoir
We present a formalism that enables the study of the non-Markovian dynamics
of a three-level ladder system in a single structured reservoir. The
three-level system is strongly coupled to a bath of reservoir modes and two
quantum excitations of the reservoir are expected. We show that the dynamics
only depends on reservoir structure functions, which are products of the mode
density with the coupling constant squared. This result may enable pseudomode
theory to treat multiple excitations of a structured reservoir. The treatment
uses Laplace transforms and an elimination of variables to obtain a formal
solution. This can be evaluated numerically (with the help of a numerical
inverse Laplace transform) and an example is given. We also compare this result
with the case where the two transitions are coupled to two separate structured
reservoirs (where the example case is also analytically solvable)
Theory of Pseudomodes in Quantum Optical Processes
This paper deals with non-Markovian behaviour in atomic systems coupled to a
structured reservoir of quantum EM field modes, with particular relevance to
atoms interacting with the field in high Q cavities or photonic band gap
materials. In cases such as the former, we show that the pseudo mode theory for
single quantum reservoir excitations can be obtained by applying the Fano
diagonalisation method to a system in which the atomic transitions are coupled
to a discrete set of (cavity) quasimodes, which in turn are coupled to a
continuum set of (external) quasimodes with slowly varying coupling constants
and continuum mode density. Each pseudomode can be identified with a discrete
quasimode, which gives structure to the actual reservoir of true modes via the
expressions for the equivalent atom-true mode coupling constants. The quasimode
theory enables cases of multiple excitation of the reservoir to now be treated
via Markovian master equations for the atom-discrete quasimode system.
Applications of the theory to one, two and many discrete quasimodes are made.
For a simple photonic band gap model, where the reservoir structure is
associated with the true mode density rather than the coupling constants, the
single quantum excitation case appears to be equivalent to a case with two
discrete quasimodes
Can Life develop in the expanded habitable zones around Red Giant Stars?
We present some new ideas about the possibility of life developing around
sub-giant and red giant stars. Our study concerns the temporal evolution of the
habitable zone. The distance between the star and the habitable zone, as well
as its width, increases with time as a consequence of stellar evolution. The
habitable zone moves outward after the star leaves the main sequence, sweeping
a wider range of distances from the star until the star reaches the tip of the
asymptotic giant branch. If life could form and evolve over time intervals from
to years, then there could be habitable planets with
life around red giant stars. For a 1 M star at the first stages of
its post main-sequence evolution, the temporal transit of the habitable zone is
estimated to be of several 10 years at 2 AU and around 10 years at 9
AU. Under these circumstances life could develop at distances in the range 2-9
AU in the environment of sub-giant or giant stars and in the far distant future
in the environment of our own Solar System. After a star completes its first
ascent along the Red Giant Branch and the He flash takes place, there is an
additional stable period of quiescent He core burning during which there is
another opportunity for life to develop. For a 1 M star there is an
additional years with a stable habitable zone in the region from 7 to 22
AU. Space astronomy missions, such as proposed for the Terrestrial Planet
Finder (TPF) and Darwin should also consider the environments of sub-giants and
red giant stars as potentially interesting sites for understanding the
development of life
Eccentricity Evolution in Simulated Galaxy Clusters
Strong cluster eccentricity evolution for has appeared in a
variety of observational data sets. We examine the evolution of eccentricity in
simulated galaxy clusters using a variety of simulation methodologies,
amplitude normalizations, and background cosmologies. We do not find find such
evolution for in any of our simulation ensembles. We suggest a
systematic error in the form of a redshift-dependent selection effect in
cluster catalogs or missing physics in cluster simulations important enough to
modify the cluster morphology.Comment: Revised version to be published in ApJ. Moderate revisions, including
additional N-body simulations with varying amplitude normalization and
background matter density within OCDM and CDM scenarios reinforce
our conclusion that observed clusters have recently relaxed much more rapidly
than simulated one
The Northern Sky Optical Cluster Survey II: An Objective Cluster Catalog for 5800 Square Degrees
We present a new, objectively defined catalog of candidate galaxy clusters
based on the galaxy catalogs from the Digitized Second Palomar Observatory Sky
Survey (DPOSS). This cluster catalog, derived from the best calibrated plates
in the high latitude (|b|>30) Northern Galactic Cap region, covers 5,800 square
degrees, and contains 8,155 candidate clusters. A simple adaptive kernel
density mapping technique, combined with the SExtractor object detection
algorithm, is used to detect galaxy overdensities, which we identify as
clusters. Simulations of the background galaxy distribution and clusters of
varying richnesses and redshifts allow us to optimize detection parameters, and
measure the completeness and contamination rates for our catalog. Cluster
richnesses and photometric redshifts are measured, using integrated colors and
magnitudes for each cluster. An extensive spectroscopic survey is used to
confirm the photometric results.
This catalog, with well-characterized sample properties, provides a sound
basis for future studies of cluster physics and large scale structure.Comment: 49 pages, 16 figures. Accepted to AJ; appearing in April. Version
with full resolution figures, and full length tables available at
http://dposs.caltech.edu:8080/NoSOCS.htm
Medication adherence during adjunct therapy with statins and ACE inhibitors in adolescents with type 1 diabetes
OBJECTIVE: Suboptimal adherence to insulin treatment is a main issue in adolescents with type 1 diabetes. However, to date, there are no available data on adherence to adjunct noninsulin medications in this population. Our aim was to assess adherence to ACE inhibitors and statins and explore potential determinants in adolescents with type 1 diabetes.
RESEARCH DESIGN AND METHODS:There were 443 adolescents with type 1 diabetes recruited into the Adolescent Type 1 Diabetes Cardio-Renal Intervention Trial (AdDIT) and exposed to treatment with two oral drugsâan ACE inhibitor and a statinâas well as combinations of both or placebo for 2â4 years. Adherence was assessed every 3 months with the Medication Event Monitoring System (MEMS) and pill count.
RESULTS: Median adherence during the trial was 80.2% (interquartile range 63.6â91.8) based on MEMS and 85.7% (72.4â92.9) for pill count. Adherence based on MEMS and pill count dropped from 92.9% and 96.3%, respectively, at the first visit to 76.3% and 79.0% at the end of the trial. The percentage of study participants with adherence âĽ75% declined from 84% to 53%. A good correlation was found between adherence based on MEMS and pill count (r = 0.82, P < 0.001). Factors associated with adherence were age, glycemic control, and country.
CONCLUSIONS: We report an overall good adherence to ACE inhibitors and statins during a clinical trial, although there was a clear decline in adherence over time. Older age and suboptimal glycemic control at baseline predicted lower adherence during the trial, and, predictably, reduced adherence was more prevalent in subjects who subsequently dropped out
Steps toward the power spectrum of matter. I.The mean spectrum of galaxies
We calculate the mean power spectrum of galaxies using published power
spectra of galaxies and clusters of galaxies. On small scales we use the power
spectrum derived from the 2-dimensional distribution of APM galaxies, on large
scales we use power spectra derived from 3-dimensional data for galaxy and
cluster samples. Spectra are reduced to real space and to the amplitude of the
power spectrum of APM galaxies. Available data indicate the presence of two
different populations in the nearby Universe. Clusters of galaxies sample a
relatively large region in the Universe where rich, medium and poor
superclusters are well represented. Their mean power spectrum has a spike on
scale 120 h^{-1}Mpc, followed by an approximate power-law spectrum of index n =
-1.9 towards small scales. The power spectrum found from LCRS and IRAS 1.2 Jy
surveys is flatter around the maximum, which may represent regions of the
Universe with medium-rich and poor superclusters.Comment: LaTex (sty files added), 35 pages, 5 PostScript figures and Table
with mean power spectrum embedded, Astrophysical Journal (accepted
A Personal Construct Psychology based investigation into a Product Service System for renting pushchairs to consumers
This is the peer-reviewed version of the following article: Maurizio Catulli and Nick Reed, âA Personal Construct Psychology Based Investigation Into a Product Service System for Renting Pushchairs to Consumersâ, Business Strategy and the Environment, Vol. 26(5): 656-671, February 2017, which has been published in final form at DOI: 10.1002/bse.1944. Under embargo. Embargo end date: 1 February 2019. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving.This paper explores how consumers construe a Product Service System (PSS) for the supply of pushchairs. A PSS is a system of products, services, networks of actors and supporting infrastructure designed to be more sustainable than traditional business models. PSS face an implementation challenge in consumer markets and this case based research explores some reasons for this. The study applies Personal Construct Psychology (in particular, Repertory Grid Technique) which has not previously been used in relation to researching PSS. Results suggest that PSS might be difficult to implement in relation to pushchairs. Renting pre-used equipment may meet resistance because of a perceived risk that acquisition by this means might endanger infants. Participants in the study construed buying new products from specialist infant product shops as being the best way of acquiring them. Accordingly PSS providers may, for instance, have to implement certified quality assurance processes in order to reassure consumers.Peer reviewedFinal Accepted Versio
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