20,872 research outputs found
On regression adjustments in experiments with several treatments
Regression adjustments are often made to experimental data. Since
randomization does not justify the models, bias is likely; nor are the usual
variance calculations to be trusted. Here, we evaluate regression adjustments
using Neyman's nonparametric model. Previous results are generalized, and more
intuitive proofs are given. A bias term is isolated, and conditions are given
for unbiased estimation in finite samples.Comment: Published in at http://dx.doi.org/10.1214/07-AOAS143 the Annals of
Applied Statistics (http://www.imstat.org/aoas/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Limits of Econometrics
In the social and behavioral sciences, far-reaching claims are often made for the superiority of advanced quantitative methods by those who manage to ignore the far-reaching assumptions behind the models. In section 2, we see there was considerable skepticism about disentangling causal processes by statistical modeling. Freedman (2005) examined several well-known modeling exercises, and discovered good reasons for skepticism. Some kinds of problems may yield to sophisticated statistical technique; others will not. The goal of empirical research is or should be to increase our understanding of the phenomena, rather than displaying our mastery of technique.
Determination of Earth orientation using the Global Positioning System
Modern spacecraft tracking and navigation require highly accurate Earth-orientation parameters. For near-real-time applications, errors in these quantities and their extrapolated values are a significant error source. A globally distributed network of high-precision receivers observing the full Global Positioning System (GPS) configuration of 18 or more satellites may be an efficient and economical method for the rapid determination of short-term variations in Earth orientation. A covariance analysis using the JPL Orbit Analysis and Simulation Software (OASIS) was performed to evaluate the errors associated with GPS measurements of Earth orientation. These GPS measurements appear to be highly competitive with those from other techniques and can potentially yield frequent and reliable centimeter-level Earth-orientation information while simultaneously allowing the oversubscribed Deep Space Network (DSN) antennas to be used more for direct project support
Homological Product Codes
Quantum codes with low-weight stabilizers known as LDPC codes have been
actively studied recently due to their simple syndrome readout circuits and
potential applications in fault-tolerant quantum computing. However, all
families of quantum LDPC codes known to this date suffer from a poor distance
scaling limited by the square-root of the code length. This is in a sharp
contrast with the classical case where good families of LDPC codes are known
that combine constant encoding rate and linear distance. Here we propose the
first family of good quantum codes with low-weight stabilizers. The new codes
have a constant encoding rate, linear distance, and stabilizers acting on at
most qubits, where is the code length. For comparison, all
previously known families of good quantum codes have stabilizers of linear
weight. Our proof combines two techniques: randomized constructions of good
quantum codes and the homological product operation from algebraic topology. We
conjecture that similar methods can produce good stabilizer codes with
stabilizer weight for any . Finally, we apply the homological
product to construct new small codes with low-weight stabilizers.Comment: 49 page
Hypervelocity atomic oxygen source for the study of atom-surface interactions
Planned improvements in an electric discharge heated atomic oxygen beam source are described which will provide 6 to 7 kms(-1) beams of atomic oxygen with a flux of 10(16) cm(-2) s(-1) at 50 cm distance from the source aperture. A major advance will be the use of a zone of silence nozzle-skimmer arrangement which is necessitated by the need for high source flux and performance. It is anticipated that a Phase 2 program would provide for the fabrication of a two stage vacuum system which would be suitable for bolting on to a UHV (ultrahigh vacuum) surface study apparatus
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