86,867 research outputs found
Error-free milestones in error prone measurements
A predictor variable or dose that is measured with substantial error may
possess an error-free milestone, such that it is known with negligible error
whether the value of the variable is to the left or right of the milestone.
Such a milestone provides a basis for estimating a linear relationship between
the true but unknown value of the error-free predictor and an outcome, because
the milestone creates a strong and valid instrumental variable. The inferences
are nonparametric and robust, and in the simplest cases, they are exact and
distribution free. We also consider multiple milestones for a single predictor
and milestones for several predictors whose partial slopes are estimated
simultaneously. Examples are drawn from the Wisconsin Longitudinal Study, in
which a BA degree acts as a milestone for sixteen years of education, and the
binary indicator of military service acts as a milestone for years of service.Comment: Published in at http://dx.doi.org/10.1214/08-AOAS233 the Annals of
Applied Statistics (http://www.imstat.org/aoas/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Non-Simply-Connected Gauge Groups and Rational Points on Elliptic Curves
We consider the F-theory description of non-simply-connected gauge groups
appearing in the E8 x E8 heterotic string. The analysis is closely tied to the
arithmetic of torsion points on an elliptic curve. The general form of the
corresponding elliptic fibration is given for all finite subgroups of E8 which
are applicable in this context. We also study the closely-related question of
point-like instantons on a K3 surface whose holonomy is a finite group. As an
example we consider the case of the heterotic string on a K3 surface having the
E8 gauge symmetry broken to (E6 x SU(3))/Z3 or SU(9)/Z3 by point-like
instantons with Z3 holonomy.Comment: 15 pages, 2 embedded figures, some spurious U(1)'s remove
The Absorption of Sound in Suspensions and Emulsions. I. Water Fog in Air
The suspended particles are approximated by spheres and the diffraction problem for a fluid sphere in a fluid medium is solved taking into consideration viscosity and thermal conduction. The results are discussed numerically for water droplets in air and a satisfactory agreement with Knudsen's attenuation measurements in water fog is found
Isolation in the construction of natural experiments
A natural experiment is a type of observational study in which treatment
assignment, though not randomized by the investigator, is plausibly close to
random. A process that assigns treatments in a highly nonrandom, inequitable
manner may, in rare and brief moments, assign aspects of treatments at random
or nearly so. Isolating those moments and aspects may extract a natural
experiment from a setting in which treatment assignment is otherwise quite
biased, far from random. Isolation is a tool that focuses on those rare, brief
instances, extracting a small natural experiment from otherwise useless data.
We discuss the theory behind isolation and illustrate its use in a reanalysis
of a well-known study of the effects of fertility on workforce participation.
Whether a woman becomes pregnant at a certain moment in her life and whether
she brings that pregnancy to term may reflect her aspirations for family,
education and career, the degree of control she exerts over her fertility, and
the quality of her relationship with the father; moreover, these aspirations
and relationships are unlikely to be recorded with precision in surveys and
censuses, and they may confound studies of workforce participation. However,
given that a women is pregnant and will bring the pregnancy to term, whether
she will have twins or a single child is, to a large extent, simply luck. Given
that a woman is pregnant at a certain moment, the differential comparison of
two types of pregnancies on workforce participation, twins or a single child,
may be close to randomized, not biased by unmeasured aspirations. In this
comparison, we find in our case study that mothers of twins had more children
but only slightly reduced workforce participation, approximately 5% less time
at work for an additional child.Comment: Published in at http://dx.doi.org/10.1214/14-AOAS770 the Annals of
Applied Statistics (http://www.imstat.org/aoas/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Reactivation of Limestone-Derived Sorbents using Hydration: Preliminary Results From a Fluidised Bed
A simple method of CO~2~ capture is by using the calcium looping cycle. The calcium looping cycle uses CaCO~3~ as a CO~2~ carrier, via the reversible reaction CaO(s) + CO~2~(g) = CaCO~3~(s), to extract CO2 from the exhaust stream and provide a pure stream of CO~2~ suitable for sequestration. 
A problem associated with the technology is that the capacity of the sorbent to absorb CO~2~ reduces significantly with the number of cycles of carbonation and calcination. The energy penalty of the cycle is considerably increased by cycling unreacted sorbent: hydration of unreactive sorbent has emerged as a promising strategy of reducing this penalty by regenerating the reactivity of exhausted sorbent.
A small atmospheric pressure fluidised bed reactor has been built and tested, that allows repeated cycling between two temperatures up to 1000 °C. 
Work presented here focuses on the effects of variation of the calcination temperature before hydration. Hydration has been found to more than double the reactivity of a spent sorbent cycled under the mildest conditions studied (calcination temperature of 840 °C). However, as calcination temperature is increased the observed reactivation decreases until little reactivation is observed for the sorbent cycled at 950 °C. The primary reason for this appears to be a substantial increase in friability of particles, with reactivity normalised for mass losses appearing similar independent of cycling temperature
Cross-screening in observational studies that test many hypotheses
We discuss observational studies that test many causal hypotheses, either
hypotheses about many outcomes or many treatments. To be credible an
observational study that tests many causal hypotheses must demonstrate that its
conclusions are neither artifacts of multiple testing nor of small biases from
nonrandom treatment assignment. In a sense that needs to be defined carefully,
hidden within a sensitivity analysis for nonrandom assignment is an enormous
correction for multiple testing: in the absence of bias, it is extremely
improbable that multiple testing alone would create an association insensitive
to moderate biases. We propose a new strategy called "cross-screening",
different from but motivated by recent work of Bogomolov and Heller on
replicability. Cross-screening splits the data in half at random, uses the
first half to plan a study carried out on the second half, then uses the second
half to plan a study carried out on the first half, and reports the more
favorable conclusions of the two studies correcting using the Bonferroni
inequality for having done two studies. If the two studies happen to concur,
then they achieve Bogomolov-Heller replicability; however, importantly,
replicability is not required for strong control of the family-wise error rate,
and either study alone suffices for firm conclusions. In randomized studies
with a few hypotheses, cross-split screening is not an attractive method when
compared with conventional methods of multiplicity control, but it can become
attractive when hundreds or thousands of hypotheses are subjected to
sensitivity analyses in an observational study. We illustrate the technique by
comparing 46 biomarkers in individuals who consume large quantities of fish
versus little or no fish.Comment: 33 pages, 2 figures, 5 table
The asset-correlation parameter in Basel II for mortgages on single-family residences
Bank capital ; Risk management ; Basel capital accord ; Mortgages
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