3,185 research outputs found
Permutation Inference for Canonical Correlation Analysis
Canonical correlation analysis (CCA) has become a key tool for population
neuroimaging, allowing investigation of associations between many imaging and
non-imaging measurements. As other variables are often a source of variability
not of direct interest, previous work has used CCA on residuals from a model
that removes these effects, then proceeded directly to permutation inference.
We show that such a simple permutation test leads to inflated error rates. The
reason is that residualisation introduces dependencies among the observations
that violate the exchangeability assumption. Even in the absence of nuisance
variables, however, a simple permutation test for CCA also leads to excess
error rates for all canonical correlations other than the first. The reason is
that a simple permutation scheme does not ignore the variability already
explained by previous canonical variables. Here we propose solutions for both
problems: in the case of nuisance variables, we show that transforming the
residuals to a lower dimensional basis where exchangeability holds results in a
valid permutation test; for more general cases, with or without nuisance
variables, we propose estimating the canonical correlations in a stepwise
manner, removing at each iteration the variance already explained, while
dealing with different number of variables in both sides. We also discuss how
to address the multiplicity of tests, proposing an admissible test that is not
conservative, and provide a complete algorithm for permutation inference for
CCA.Comment: 49 pages, 2 figures, 10 tables, 3 algorithms, 119 reference
Static inverters which sum a plurality of waves Patent
Describing static inverter with single or multiple phase outpu
Recommended from our members
Meteorological impacts of the total solar eclipse of 21 August 2017
A solar eclipse presents many opportunities for examining the impact of the reduction in solar radiation upon meteorological conditions, particularly so as accurate predictions of the circumstances and extent of the eclipse well before the event enable bespoke observing programmes to be set out in advance. The total solar eclipse of 21 August 2017 traversed the United States from north-west to south-east (Figure 1) and a partial eclipse (at least) was visible in every state. According to news reports, the path of totality was lined with more than 10 million people, making this probably the largest audience for any total solar eclipse in human history. The time of year and time of day was favourable for both good viewing conditions along much of the eclipse path, and for the detection of eclipse-related impacts on surface meteorological variables
Tobacco Control Progress in Low and Middle Income Countries in Comparison to High Income Countries
The study aimed to describe worldwide levels and trends of tobacco control
policy by comparing low and middle income countries with other income
categories from 2007 to 2014 and to analyze the corresponding relation to
recent changes in smoking prevalence. Policy measure data representing years
2007 to 2014 were collected from all available World Health Organization (WHO)
reports on the global tobacco epidemic. Corresponding policy percentage scores
(PS) were calculated based on MPOWER measures. Age-standardized smoking
prevalence data for years 2010 and 2015 were collected from the WHO Global
Health Observatory Data Repository. Trends of PS were analysed with respect to
WHO region and OECD country income category. Scatter plots and regression
analysis were used to depict the relationship between tobacco control policy
of 2010 and change in smoking prevalence between 2015 and 2010 by sex and
income category. Combined PS for all countries increased significantly from
47% in 2007 to 61% by 2014 (p < 0.001). When grouped by income category and
region, policies were strengthened in all categories, albeit with varying
progression. By 2014, tobacco control policy legislation had reached 45% in
the Least Developed Countries (LDCs), 59% in Low Middle Income Countries
(LMICs), 66% in Upper Middle Income Countries (UMICs) and 70% in High Income
Countries (HICs). Overall, there was a negative relationship between higher
policy scores and change in smoking prevalence. Although policy strengthening
had been conducted between 2007 and 2014, room for considerable global
improvement remains, particularly in LDCs. View Full-Tex
Standing wave oscillations in binary mixture convection: from onset via symmetry breaking to period doubling into chaos
Oscillatory solution branches of the hydrodynamic field equations describing
convection in the form of a standing wave (SW) in binary fluid mixtures heated
from below are determined completely for several negative Soret coefficients.
Galerkin as well as finite-difference simulations were used. They were
augmented by simple control methods to obtain also unstable SW states. For
sufficiently negative Soret coefficients unstable SWs bifurcate subcritically
out of the quiescent conductive state. They become stable via a saddle-node
bifurcation when lateral phase pinning is exerted. Eventually their invariance
under time-shift by half a period combined with reflexion at midheight of the
fluid layer gets broken. Thereafter they terminate by undergoing a
period-doubling cascade into chaos
False Discovery Rate and Localizing Power
False discovery rate (FDR) is commonly used for correction for multiple testing in neuroimaging studies. However, when using two-tailed tests, making directional inferences about the results can lead to vastly inflated error rate, even approaching 100% in some cases. This happens because FDR only provides weak control over the error rate, meaning that the proportion of error is guaranteed only globally over all tests, not within subsets, such as among those in only one or another direction. Here we consider and evaluate different strategies for FDR control with two-tailed tests, using both synthetic and real imaging data. Approaches that separate the tests by direction of the hypothesis test, or by the direction of the resulting test statistic, more properly control the directional error rate and preserve FDR benefits, albeit with a doubled risk of errors under complete absence of signal. Strategies that combine tests in both directions, or that use simple two-tailed p-values, can lead to invalid directional conclusions, even if these tests remain globally valid. To enable valid thresholding for directional inference, we suggest that imaging software should allow the possibility that the user sets asymmetrical thresholds for the two sides of the statistical map. While FDR continues to be a valid, powerful procedure for multiple testing correction, care is needed when making directional inferences for two-tailed tests, or more broadly, when making any localized inference
Non-spiky density of states of an icosahedral quasicrystal
The density of states of the ideal three-dimensional Penrose tiling, a
quasicrystalline model, is calculated with a resolution of 10 meV. It is not
spiky. This falsifies theoretical predictions so far, that spikes of width
10-20 meV are generic for the density of states of quasicrystals, and it
confirms recent experimental findings. The qualitative difference between our
results and previous calculations is partly explained by the small number of k
points that has usually been included in the evaluation of the density of
states of periodic approximants of quasicrystals. It is also shown that both
the density of states of a small approximant of the three-dimensional Penrose
tiling and the density of states of the ideal two-dimensional Penrose tiling do
have spiky features, which also partly explains earlier predictions.Comment: 8 pages, 4 figures. Changes in this version: longer introduction,
details of figures shown in inset
Wavefunction considerations for the central spin decoherence problem in a nuclear spin bath
Decoherence of a localized electron spin in a solid state material (the
``central spin'' problem) at low temperature is believed to be dominated by
interactions with nuclear spins in the lattice. This decoherence is partially
suppressed through the application of a large magnetic field that splits the
energy levels of the electron spin and prevents depolarization. However,
dephasing decoherence resulting from a dynamical nuclear spin bath cannot be
removed in this way. Fluctuations of the nuclear field lead to uncertainty of
the electron's precessional frequency in a process known as spectral diffusion.
This article considers the effect of the electron's wavefunction shape upon
spectral diffusion and provides wavefunction dependent decoherence time
formulas for free induction decay as well as spin echoes and concatenated
dynamical decoupling schemes for enhancing coherence. We also discuss dephasing
of a qubit encoded in singlet-triplet states of a double quantum dot. A central
theoretical result of this work is the development of a continuum approximation
for the spectral diffusion problem which we have applied to GaAs and InAs
materials specifically
Mega-analysis methods in ENIGMA : the experience of the generalized anxiety disorder working group
The ENIGMA group on Generalized Anxiety Disorder (ENIGMA-Anxiety/GAD) is part of a broader effort to investigate anxiety disorders using imaging and genetic data across multiple sites worldwide. The group is actively conducting a mega-analysis of a large number of brain structural scans. In this process, the group was confronted with many methodological challenges related to study planning and implementation, between-country transfer of subject-level data, quality control of a considerable amount of imaging data, and choices related to statistical methods and efficient use of resources. This report summarizes the background information and rationale for the various methodological decisions, as well as the approach taken to implement them. The goal is to document the approach and help guide other research groups working with large brain imaging data sets as they develop their own analytic pipelines for mega-analyses
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