3,885 research outputs found
Statistical modeling of causal effects in continuous time
This article studies the estimation of the causal effect of a time-varying
treatment on time-to-an-event or on some other continuously distributed
outcome. The paper applies to the situation where treatment is repeatedly
adapted to time-dependent patient characteristics. The treatment effect cannot
be estimated by simply conditioning on these time-dependent patient
characteristics, as they may themselves be indications of the treatment effect.
This time-dependent confounding is common in observational studies. Robins
[(1992) Biometrika 79 321--334, (1998b) Encyclopedia of Biostatistics 6
4372--4389] has proposed the so-called structural nested models to estimate
treatment effects in the presence of time-dependent confounding. In this
article we provide a conceptual framework and formalization for structural
nested models in continuous time. We show that the resulting estimators are
consistent and asymptotically normal. Moreover, as conjectured in Robins
[(1998b) Encyclopedia of Biostatistics 6 4372--4389], a test for whether
treatment affects the outcome of interest can be performed without specifying a
model for treatment effect. We illustrate the ideas in this article with an
example.Comment: Published in at http://dx.doi.org/10.1214/009053607000000820 the
Annals of Statistics (http://www.imstat.org/aos/) by the Institute of
Mathematical Statistics (http://www.imstat.org
The survival-incorporated median versus the median in the survivors or in the always-survivors: What are we measuring? And why?
Many clinical studies evaluate the benefit of treatment based on both
survival and other ordinal/continuous clinical outcomes, such as neurocognitive
scores or quality-of-life scores. In these studies, there are situations when
the clinical outcomes are truncated by death, where subjects die before their
clinical outcome is measured. Treating outcomes as "missing" or "censored" due
to death can be misleading for treatment effect evaluation. We show that if we
use the median in the survivors or in the always-survivors to summarize
clinical outcomes, we may conclude a trade-off exists between the probability
of survival and good clinical outcomes, even in settings where both the
probability of survival and the probability of any good clinical outcome are
better for one treatment. Therefore, we advocate not always treating death as a
mechanism through which clinical outcomes are missing, but rather as part of
the outcome measure. To account for the survival status, we describe the
survival-incorporated median as an alternative summary measure for outcomes in
the presence of death. The survival-incorporated median is the threshold such
that 50\% of the population is alive with an outcome above that threshold. We
use conceptual examples to show that the survival-incorporated median provides
a simple and useful summary measure to inform clinical practice
Spin-phonon coupling in single Mn doped CdTe quantum dot
The spin dynamics of a single Mn atom in a laser driven CdTe quantum dot is
addressed theoretically. Recent experimental
results\cite{Le-Gall_PRL_2009,Goryca_PRL_2009,Le-Gall_PRB_2010}show that it is
possible to induce Mn spin polarization by means of circularly polarized
optical pumping. Pumping is made possible by the faster Mn spin relaxation in
the presence of the exciton. Here we discuss different Mn spin relaxation
mechanisms. First, Mn-phonon coupling, which is enhanced in the presence of the
exciton. Second, phonon-induced hole spin relaxation combined with carrier-Mn
spin flip coupling and photon emission results in Mn spin relaxation. We model
the Mn spin dynamics under the influence of a pumping laser that injects
excitons into the dot, taking into account exciton-Mn exchange and phonon
induced spin relaxation of both Mn and holes. Our simulations account for the
optically induced Mn spin pumping.Comment: 17 pages, 11 figures, submitted to PR
Electron spin resonance on a 2-dimensional electron gas in a single AlAs quantum well
Direct electron spin resonance (ESR) on a high mobility two dimensional
electron gas in a single AlAs quantum well reveals an electronic -factor of
1.991 at 9.35 GHz and 1.989 at 34 GHz with a minimum linewidth of 7 Gauss. The
ESR amplitude and its temperature dependence suggest that the signal originates
from the effective magnetic field caused by the spin orbit-interaction and a
modulation of the electron wavevector caused by the microwave electric field.
This contrasts markedly to conventional ESR that detects through the microwave
magnetic field.Comment: 4 pages, 4 figure
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