820 research outputs found
Estimating the Proportion of True Null Hypotheses for Multiple Comparisons
Whole genome microarray investigations (e.g. differential expression, differential methylation, ChIP-Chip) provide opportunities to test millions of features in a genome. Traditional multiple comparison procedures such as familywise error rate (FWER) controlling procedures are too conservative. Although false discovery rate (FDR) procedures have been suggested as having greater power, the control itself is not exact and depends on the proportion of true null hypotheses. Because this proportion is unknown, it has to be accurately (small bias, small variance) estimated, preferably using a simple calculation that can be made accessible to the general scientific community. We propose an easy-to-implement method and make the R code available, for estimating the proportion of true null hypotheses. This estimate has relatively small bias and small variance as demonstrated by (simulated and real data) comparing it with four existing procedures. Although presented here in the context of microarrays, this estimate is applicable for many multiple comparison situations
Trajectory and stability of Lagrangian point in the Sun-Earth system
This paper describes design of the trajectory and analysis of the stability
of collinear point in the Sun-Earth system. The modified restricted three
body problem with additional gravitational potential from the belt is used as
the model for the Sun-Earth system. The effect of radiation pressure of the Sun
and oblate shape of the Earth are considered. The point is asymptotically
stable upto a specific value of time correspond to each set of values of
parameters and initial conditions. The results obtained from this study would
be applicable to locate a satellite, a telescope or a space station around the
point .Comment: Accepted for publication in Astrophysics & Space Scienc
Uniform random generation of large acyclic digraphs
Directed acyclic graphs are the basic representation of the structure
underlying Bayesian networks, which represent multivariate probability
distributions. In many practical applications, such as the reverse engineering
of gene regulatory networks, not only the estimation of model parameters but
the reconstruction of the structure itself is of great interest. As well as for
the assessment of different structure learning algorithms in simulation
studies, a uniform sample from the space of directed acyclic graphs is required
to evaluate the prevalence of certain structural features. Here we analyse how
to sample acyclic digraphs uniformly at random through recursive enumeration,
an approach previously thought too computationally involved. Based on
complexity considerations, we discuss in particular how the enumeration
directly provides an exact method, which avoids the convergence issues of the
alternative Markov chain methods and is actually computationally much faster.
The limiting behaviour of the distribution of acyclic digraphs then allows us
to sample arbitrarily large graphs. Building on the ideas of recursive
enumeration based sampling we also introduce a novel hybrid Markov chain with
much faster convergence than current alternatives while still being easy to
adapt to various restrictions. Finally we discuss how to include such
restrictions in the combinatorial enumeration and the new hybrid Markov chain
method for efficient uniform sampling of the corresponding graphs.Comment: 15 pages, 2 figures. To appear in Statistics and Computin
Coexistence of ferro- and antiferromagnetic order in Mn-doped NiMnGa
Ni-Mn-Ga is interesting as a prototype of a magnetic shape-memory alloy
showing large magnetic field induced strains. We present here results for the
magnetic ordering of Mn-rich Ni-Mn-Ga alloys based on both experiments and
theory. Experimental trends for the composition dependence of the magnetization
are measured by a vibrating sample magnetometer (VSM) in magnetic fields of up
to several tesla and at low temperatures. The saturation magnetization has a
maximum near the stoichiometric composition and it decreases with increasing Mn
content. This unexpected behaviour is interpreted via first-principles
calculations within the density-functional theory. We show that extra Mn atoms
are antiferromagnetically aligned to the other moments, which explains the
dependence of the magnetization on composition. In addition, the effect of Mn
doping on the stabilization of the structural phases and on the magnetic
anisotropy energy is demonstrated.Comment: 4 pages, 3 figure
Periodic and Quasiperiodic Motion of an Elongated Microswimmer in Poiseuille Flow
We study the dynamics of a prolate spheroidal microswimmer in Poiseuille flow
for different flow geometries. When moving between two parallel plates or in a
cylindrical microchannel, the swimmer performs either periodic swinging or
periodic tumbling motion. Although the trajectories of spherical and elongated
swimmers are qualitatively similar, the swinging and tumbling frequency
strongly depends on the aspect ratio of the swimmer. In channels with reduced
symmetry the swimmers perform quasiperiodic motion which we demonstrate
explicitely for swimming in a channel with elliptical cross section
Predicting the Amplitude of a Solar Cycle Using the North-South Asymmetry in the Previous Cycle: II. An Improved Prediction for Solar Cycle~24
Recently, using Greenwich and Solar Optical Observing Network sunspot group
data during the period 1874-2006, (Javaraiah, MNRAS, 377, L34, 2007: Paper I),
has found that: (1) the sum of the areas of the sunspot groups in 0-10 deg
latitude interval of the Sun's northern hemisphere and in the time-interval of
-1.35 year to +2.15 year from the time of the preceding minimum of a solar
cycle n correlates well (corr. coeff. r=0.947) with the amplitude (maximum of
the smoothed monthly sunspot number) of the next cycle n+1. (2) The sum of the
areas of the spot groups in 0-10 deg latitude interval of the southern
hemisphere and in the time-interval of 1.0 year to 1.75 year just after the
time of the maximum of the cycle n correlates very well (r=0.966) with the
amplitude of cycle n+1. Using these relations, (1) and (2), the values 112 + or
- 13 and 74 + or -10, respectively, were predicted in Paper I for the amplitude
of the upcoming cycle 24. Here we found that in case of (1), the north-south
asymmetry in the area sum of a cycle n also has a relationship, say (3), with
the amplitude of cycle n+1, which is similar to (1) but more statistically
significant (r=0.968) like (2). By using (3) it is possible to predict the
amplitude of a cycle with a better accuracy by about 13 years in advance, and
we get 103 + or -10 for the amplitude of the upcoming cycle 24. However, we
found a similar but a more statistically significant (r=0.983) relationship,
say (4), by using the sum of the area sum used in (2) and the north-south
difference used in (3). By using (4) it is possible to predict the amplitude of
a cycle by about 9 years in advance with a high accuracy and we get 87 + or - 7
for the amplitude of cycle 24.Comment: 21 pages, 7 figures, Published in Solar Physics 252, 419-439 (2008
Recoil Polarization for Delta Excitation in Pion Electroproduction
We measured angular distributions of recoil-polarization response functions
for neutral pion electroproduction for W=1.23 GeV at Q^2=1.0 (GeV/c)^2,
obtaining 14 separated response functions plus 2 Rosenbluth combinations; of
these, 12 have been observed for the first time. Dynamical models do not
describe quantities governed by imaginary parts of interference products well,
indicating the need for adjusting magnitudes and phases for nonresonant
amplitudes. We performed a nearly model-independent multipole analysis and
obtained values for Re(S1+/M1+)=-(6.84+/-0.15)% and Re(E1+/M1+)=-(2.91+/-0.19)%
that are distinctly different from those from the traditional Legendre analysis
based upon M1+ dominance and sp truncation.Comment: 5 pages, 2 figures, for PR
Population statistics study of radio and gamma-ray pulsars in the Galactic plane
We present results of our pulsar population synthesis of ordinary isolated
and millisecond pulsars in the Galactic plane. Over the past several years, a
program has been developed to simulate pulsar birth, evolution and emission
using Monte Carlo techniques. We have added to the program the capability to
simulate millisecond pulsars, which are old, recycled pulsars with extremely
short periods. We model the spatial distribution of the simulated pulsars by
assuming that they start with a random kick velocity and then evolve through
the Galactic potential. We use a polar cap/slot gap model for -ray
emission from both millisecond and ordinary pulsars. From our studies of radio
pulsars that have clearly identifiable core and cone components, in which we
fit the polarization sweep as well as the pulse profiles in order to constrain
the viewing geometry, we develop a model describing the ratio of radio
core-to-cone peak fluxes. In this model, short period pulsars are more
cone-dominated than in our previous studies. We present the preliminary results
of our recent study and the implications for observing these pulsars with GLAST
and AGILE.Comment: 6 pages, 3 figures, 1 table, accepted in Astrophysics and Space
Scienc
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