20,365 research outputs found
Replication, Communication, and the Population Dynamics of Scientific Discovery
Many published research results are false, and controversy continues over the
roles of replication and publication policy in improving the reliability of
research. Addressing these problems is frustrated by the lack of a formal
framework that jointly represents hypothesis formation, replication,
publication bias, and variation in research quality. We develop a mathematical
model of scientific discovery that combines all of these elements. This model
provides both a dynamic model of research as well as a formal framework for
reasoning about the normative structure of science. We show that replication
may serve as a ratchet that gradually separates true hypotheses from false, but
the same factors that make initial findings unreliable also make replications
unreliable. The most important factors in improving the reliability of research
are the rate of false positives and the base rate of true hypotheses, and we
offer suggestions for addressing each. Our results also bring clarity to verbal
debates about the communication of research. Surprisingly, publication bias is
not always an obstacle, but instead may have positive impacts---suppression of
negative novel findings is often beneficial. We also find that communication of
negative replications may aid true discovery even when attempts to replicate
have diminished power. The model speaks constructively to ongoing debates about
the design and conduct of science, focusing analysis and discussion on precise,
internally consistent models, as well as highlighting the importance of
population dynamics
Agglomeration, Integration and Tax Harmonization
We show that agglomeration forces can reverse standard international-tax-competition results. Closer integration may result first in a race to the top' and then a race to the bottom, a result that is consistent with recent empirical work showing that the tax gap between rich and poor nations follows a bell-shaped path (Devereux, Griffith and Klemm 2002). Moreover, split-the-difference tax harmonization can make both nations worse off. This may help explain why tax harmonisation which is Pareto improving in the standard model is so difficult in the real world. The key theoretical insight is that agglomeration forces create quasi-rents that can be taxed without inducing delocation. This suggests that the tax game is something subtler than a race to the bottom. Advanced 'core' nations may act like limit-pricing monopolists toward less advanced 'periphery' countries. Since agglomeration rents are a bell-shaped function of the level of integration, the equilibrium tax gap in our tax game is also bell shaped.
The Persistence of the U.S. Trade Deficit
macroeconomics, trade deficit, dollar
LES of an Inclined Jet into a Supersonic Turbulent Crossflow
This short article describes flow parameters, numerical method, and
animations of the fluid dynamics video "LES of an Inclined Jet into a
Supersonic Turbulent Crossflow"
(http://ecommons.library.cornell.edu/bitstream/1813/14073/3/GFM-2009.mpg
[high-resolution] and
http://ecommons.library.cornell.edu/bitstream/1813/14073/2/GFM-2009-web.m1v
[low-resolution] video). We performed large-eddy simulation with the sub-grid
scale (LES-SGS) stretched-vortex model of momentum and scalar transport to
study the gas-dynamics interactions of a helium inclined round jet into a
supersonic () turbulent (\Reth) air flow over a flat
surface. The video shows the temporal development of Mach-number and magnitude
of density-gradient in the mid-span plane, and isosurface of helium
mass-fraction and \lam_2 (vortical structures). The identified vortical
structures are sheets, tilted tubes, and discontinuous rings. The vortical
structures are shown to be well correlated in space and time with helium
mass-fraction isosurface ().Comment: 7 pages, 1 figure, 1 table, article describing fluid dynamics video
submitted to Gallery of Fluid Motion, APS-DFD 200
Using nonequilibrium fluctuation theorems to understand and correct errors in equilibrium and nonequilibrium discrete Langevin dynamics simulations
Common algorithms for computationally simulating Langevin dynamics must
discretize the stochastic differential equations of motion. These resulting
finite time step integrators necessarily have several practical issues in
common: Microscopic reversibility is violated, the sampled stationary
distribution differs from the desired equilibrium distribution, and the work
accumulated in nonequilibrium simulations is not directly usable in estimators
based on nonequilibrium work theorems. Here, we show that even with a
time-independent Hamiltonian, finite time step Langevin integrators can be
thought of as a driven, nonequilibrium physical process. Once an appropriate
work-like quantity is defined -- here called the shadow work -- recently
developed nonequilibrium fluctuation theorems can be used to measure or correct
for the errors introduced by the use of finite time steps. In particular, we
demonstrate that amending estimators based on nonequilibrium work theorems to
include this shadow work removes the time step dependent error from estimates
of free energies. We also quantify, for the first time, the magnitude of
deviations between the sampled stationary distribution and the desired
equilibrium distribution for equilibrium Langevin simulations of solvated
systems of varying size. While these deviations can be large, they can be
eliminated altogether by Metropolization or greatly diminished by small
reductions in the time step. Through this connection with driven processes,
further developments in nonequilibrium fluctuation theorems can provide
additional analytical tools for dealing with errors in finite time step
integrators.Comment: 11 pages, 4 figure
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