14,645 research outputs found
Revisiting nested group testing procedures: new results, comparisons, and robustness
Group testing has its origin in the identification of syphilis in the US army
during World War II. Much of the theoretical framework of group testing was
developed starting in the late 1950s, with continued work into the 1990s.
Recently, with the advent of new laboratory and genetic technologies, there has
been an increasing interest in group testing designs for cost saving purposes.
In this paper, we compare different nested designs, including Dorfman, Sterrett
and an optimal nested procedure obtained through dynamic programming. To
elucidate these comparisons, we develop closed-form expressions for the optimal
Sterrett procedure and provide a concise review of the prior literature for
other commonly used procedures. We consider designs where the prevalence of
disease is known as well as investigate the robustness of these procedures when
it is incorrectly assumed. This article provides a technical presentation that
will be of interest to researchers as well as from a pedagogical perspective.
Supplementary material for this article is available online.Comment: Submitted for publication on May 3, 2016. Revised versio
An approach for jointly modeling multivariate longitudinal measurements and discrete time-to-event data
In many medical studies, patients are followed longitudinally and interest is
on assessing the relationship between longitudinal measurements and time to an
event. Recently, various authors have proposed joint modeling approaches for
longitudinal and time-to-event data for a single longitudinal variable. These
joint modeling approaches become intractable with even a few longitudinal
variables. In this paper we propose a regression calibration approach for
jointly modeling multiple longitudinal measurements and discrete time-to-event
data. Ideally, a two-stage modeling approach could be applied in which the
multiple longitudinal measurements are modeled in the first stage and the
longitudinal model is related to the time-to-event data in the second stage.
Biased parameter estimation due to informative dropout makes this direct
two-stage modeling approach problematic. We propose a regression calibration
approach which appropriately accounts for informative dropout. We approximate
the conditional distribution of the multiple longitudinal measurements given
the event time by modeling all pairwise combinations of the longitudinal
measurements using a bivariate linear mixed model which conditions on the event
time. Complete data are then simulated based on estimates from these pairwise
conditional models, and regression calibration is used to estimate the
relationship between longitudinal data and time-to-event data using the
complete data. We show that this approach performs well in estimating the
relationship between multivariate longitudinal measurements and the
time-to-event data and in estimating the parameters of the multiple
longitudinal process subject to informative dropout. We illustrate this
methodology with simulations and with an analysis of primary biliary cirrhosis
(PBC) data.Comment: Published in at http://dx.doi.org/10.1214/10-AOAS339 the Annals of
Applied Statistics (http://www.imstat.org/aoas/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Experimental results for a two-dimensional supersonic inlet used as a thrust deflecting nozzle
Nearly all supersonic V/STOL aircraft concepts are dependent on the thrust deflecting capability of a nozzle. In one unique concept, referred to as the reverse flow dual fan, not only is there a thrust deflecting nozzle for the fan and core engine exit flow, but because of the way the propulsion system operates during vertical takeoff and landing, the supersonic inlet is also used as a thrust deflecting nozzle. This paper presents results of an experimental study to evaluate the performance of a supersonic inlet used as a thrust deflecting nozzle for this reverse flow dual fan concept. Results are presented in terms of nozzle thrust coefficient and thrust vector angle for a number of inlet/nozzle configurations. Flow visualization and nozzle exit flow survey results are also shown
Cognitive Skills, Non-Cognitive Skills, and the Employment and Wages of Young Adults in Rural China
The objective of this paper is to examine whether noncognitive skills explain differences in employment status and hourly wages even after controlling for age, experience, schooling and cognitive skills. Of particular interest is to examine the relative magnitudes of the impacts of the cognitive and noncognitive skills on these labor market outcomes. Data used in this paper come from the Gansu Survey of Children and Families (GSCF), which followed a random sample of 2,000 children in rural areas of Gansu Province who were 9-12 years old in the year 2000. Three waves of surveys were completed in 2000, 2004, and 2007-2009. The GSCF is the first large-scale data collection on child and adolescent cognitive and noncognitive skills in rural China.cognitive skills, noncognitive skills, years of schooling, wage, Gansu, China, International Development, Labor and Human Capital,
Marginal analysis of longitudinal count data in long sequences: Methods and applications to a driving study
Most of the available methods for longitudinal data analysis are designed and
validated for the situation where the number of subjects is large and the
number of observations per subject is relatively small. Motivated by the
Naturalistic Teenage Driving Study (NTDS), which represents the exact opposite
situation, we examine standard and propose new methodology for marginal
analysis of longitudinal count data in a small number of very long sequences.
We consider standard methods based on generalized estimating equations, under
working independence or an appropriate correlation structure, and find them
unsatisfactory for dealing with time-dependent covariates when the counts are
low. For this situation, we explore a within-cluster resampling (WCR) approach
that involves repeated analyses of random subsamples with a final analysis that
synthesizes results across subsamples. This leads to a novel WCR method which
operates on separated blocks within subjects and which performs better than all
of the previously considered methods. The methods are applied to the NTDS data
and evaluated in simulation experiments mimicking the NTDS.Comment: Published in at http://dx.doi.org/10.1214/11-AOAS507 the Annals of
Applied Statistics (http://www.imstat.org/aoas/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Probing Ultrafast Dynamics with Time-resolved Multi-dimensional Coincidence Imaging: Butadiene
Time-resolved coincidence imaging of photoelectrons and photoions represents
the most complete experimental measurement of ultrafast excited state dynamics,
a multi-dimensional measurement for a multi-dimensional problem. Here we
present the experimental data from recent coincidence imaging experiments,
undertaken with the aim of gaining insight into the complex ultrafast
excited-state dynamics of 1,3-butadiene initiated by absorption of 200 nm
light. We discuss photoion and photoelectron mappings of increasing
dimensionality, and focus particularly on the time-resolved photoelectron
angular distributions (TRPADs), expected to be a sensitive probe of the
electronic evolution of the excited state and to provide significant
information beyond the time-resolved photoelectron spectrum (TRPES). Complex
temporal behaviour is observed in the TRPADs, revealing their sensitivity to
the dynamics while also emphasising the difficulty of interpretation of these
complex observables. From the experimental data some details of the wavepacket
dynamics are discerned relatively directly, and we make some tentative
comparisons with existing ab initio calculations in order to gain deeper
insight into the experimental measurements; finally, we sketch out some
considerations for taking this comparison further in order to bridge the gap
between experiment and theory.Comment: 18 pages, 10 figures. Pre-print of JMO submissio
Dipole Oscillations of a Bose-Einstein Condensate in Presence of Defects and Disorder
We consider dipole oscillations of a trapped dilute Bose-Einstein condensate
in the presence of a scattering potential consisting either in a localized
defect or in an extended disordered potential. In both cases the breaking of
superfluidity and the damping of the oscillations are shown to be related to
the appearance of a nonlinear dissipative flow. At supersonic velocities the
flow becomes asymptotically dissipationless.Comment: 4 pages, 4 figure
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