6,979 research outputs found
Inference with interference between units in an fMRI experiment of motor inhibition
An experimental unit is an opportunity to randomly apply or withhold a
treatment. There is interference between units if the application of the
treatment to one unit may also affect other units. In cognitive neuroscience, a
common form of experiment presents a sequence of stimuli or requests for
cognitive activity at random to each experimental subject and measures
biological aspects of brain activity that follow these requests. Each subject
is then many experimental units, and interference between units within an
experimental subject is likely, in part because the stimuli follow one another
quickly and in part because human subjects learn or become experienced or
primed or bored as the experiment proceeds. We use a recent fMRI experiment
concerned with the inhibition of motor activity to illustrate and further
develop recently proposed methodology for inference in the presence of
interference. A simulation evaluates the power of competing procedures.Comment: Published by Journal of the American Statistical Association at
http://www.tandfonline.com/doi/full/10.1080/01621459.2012.655954 . R package
cin (Causal Inference for Neuroscience) implementing the proposed method is
freely available on CRAN at https://CRAN.R-project.org/package=ci
Conductivity of Metallic Si:B near the Metal-Insulator Transition: Comparison between Unstressed and Uniaxially Stressed Samples
The low-temperature dc conductivities of barely metallic samples of p-type
Si:B are compared for a series of samples with different dopant concentrations,
n, in the absence of stress (cubic symmetry), and for a single sample driven
from the metallic into the insulating phase by uniaxial compression, S. For all
values of temperature and stress, the conductivity of the stressed sample
collapses onto a single universal scaling curve. The scaling fit indicates that
the conductivity of si:B is proportional to the square-root of T in the
critical range. Our data yield a critical conductivity exponent of 1.6,
considerably larger than the value reported in earlier experiments where the
transition was crossed by varying the dopant concentration. The larger exponent
is based on data in a narrow range of stress near the critical value within
which scaling holds. We show explicitly that the temperature dependences of the
conductivity of stressed and unstressed Si:B are different, suggesting that a
direct comparison of the critical behavior and critical exponents for stress-
tuned and concentration-tuned transitions may not be warranted
Percolating through networks of random thresholds: Finite temperature electron tunneling in metal nanocrystal arrays
We investigate how temperature affects transport through large networks of
nonlinear conductances with distributed thresholds. In monolayers of
weakly-coupled gold nanocrystals, quenched charge disorder produces a range of
local thresholds for the onset of electron tunneling. Our measurements
delineate two regimes separated by a cross-over temperature . Up to
the nonlinear zero-temperature shape of the current-voltage curves survives,
but with a threshold voltage for conduction that decreases linearly with
temperature. Above the threshold vanishes and the low-bias conductance
increases rapidly with temperature. We develop a model that accounts for these
findings and predicts .Comment: 5 pages including 3 figures; replaced 3/30/04: minor changes; final
versio
Fairness in Algorithmic Decision Making: An Excursion Through the Lens of Causality
As virtually all aspects of our lives are increasingly impacted by
algorithmic decision making systems, it is incumbent upon us as a society to
ensure such systems do not become instruments of unfair discrimination on the
basis of gender, race, ethnicity, religion, etc. We consider the problem of
determining whether the decisions made by such systems are discriminatory,
through the lens of causal models. We introduce two definitions of group
fairness grounded in causality: fair on average causal effect (FACE), and fair
on average causal effect on the treated (FACT). We use the Rubin-Neyman
potential outcomes framework for the analysis of cause-effect relationships to
robustly estimate FACE and FACT. We demonstrate the effectiveness of our
proposed approach on synthetic data. Our analyses of two real-world data sets,
the Adult income data set from the UCI repository (with gender as the protected
attribute), and the NYC Stop and Frisk data set (with race as the protected
attribute), show that the evidence of discrimination obtained by FACE and FACT,
or lack thereof, is often in agreement with the findings from other studies. We
further show that FACT, being somewhat more nuanced compared to FACE, can yield
findings of discrimination that differ from those obtained using FACE.Comment: 7 pages, 2 figures, 2 tables.To appear in Proceedings of the
International Conference on World Wide Web (WWW), 201
Tensor Coordinates in Noncommutative Mechanics
A consistent classical mechanics formulation is presented in such a way that,
under quantization, it gives a noncommutative quantum theory with interesting
new features. The Dirac formalism for constrained Hamiltonian systems is
strongly used, and the object of noncommutativity plays
a fundamental rule as an independent quantity. The presented classical theory,
as its quantum counterpart, is naturally invariant under the rotation group
.Comment: 12 pages, Late
High Resolution Study of Magnetic Ordering at Absolute Zero
High fidelity pressure measurements in the zero temperature limit provide a
unique opportunity to study the behavior of strongly interacting, itinerant
electrons with coupled spin and charge degrees of freedom. Approaching the
exactitude that has become the hallmark of experiments on classical critical
phenomena, we characterize the quantum critical behavior of the model,
elemental antiferromagnet chromium, lightly doped with vanadium. We resolve the
sharp doubling of the Hall coefficient at the quantum critical point and trace
the dominating effects of quantum fluctuations up to surprisingly high
temperatures.Comment: 5 pages, 4 figure
Estimation of Treatment Effect of Asthma Case Management Using Propensity Score Methods
Microscopic and Macroscopic Signatures of Antiferromagnetic Domain Walls
Magnetotransport measurements on small single crystals of Cr, the elemental
antiferromagnet, reveal the hysteretic thermodynamics of the domain structure.
The temperature dependence of the transport coefficients is directly correlated
with the real-space evolution of the domain configuration as recorded by x-ray
microprobe imaging, revealing the effect of antiferromagnetic domain walls on
electron transport. A single antiferromagnetic domain wall interface resistance
is deduced to be of order at a
temperature of 100 K.Comment: 3 color figure
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