2,502 research outputs found
Causal Inference with Two-Stage Logistic Regression - Accuracy, Precision, and Application
Two-stage predictor substitution (2SPS) and the two-stage residual inclusion (2SRI) are two approaches to instrumental variable (IV) analysis. While 2SPS and 2SRI with linear models are well-studied methods of causal inference, the properties of 2SPS and 2SRI for logistic binary outcomes have not been thoroughly studied. We study the bias and variance properties of 2SPS and 2SRI for a logistic outcome model so that we can apply these IV approaches to the causal inference of binary outcomes. We also propose and implement an extension of generalized structure mean model originally developed for a randomized trial. We first present closed form expressions of asymptotic bias for the causal odds ratio from both 2SPS and 2SRI approaches. Our closed form bias results show that the 2SPS logistic regression generates asymptotically biased estimates of this causal odds ratio when there is no unmeasured confounding and that this bias increases with increasing unmeasured confounding. The 2SRI logistic regression is asymptotically unbiased when there is no unmeasured confounding, but when there is unmeasured confounding, there is bias and it increases with increasing unmeasured confounding. In the second part, we propose the sandwich variance estimator of logistic regression of both 2SPS and 2SRI approaches and the variance estimator is adjusted for the fact that the estimates from the first stage regression is included as covariates in the second stage regression. The simulation results show that the adjusted estimates are consistent with the observed variance while the naive estimates without the adjustments are biased. This study also shows that the 2SRI method has a larger variance than the 2SPS method. Lastly, we compare the 2SPS and 2SRI logistic regression with the generalized structure mean model (GSMM). Our simulation results show that the GSMM is an unbiased estimator of complier-average causal effect (CACE) and has the least variance among the three approaches. We apply these three methods to the analysis of the GPRD database on antidiabetic effect of bezafibrate
Perturbative corrections to form factors in QCD
We compute perturbative QCD corrections to form factors at leading
power in , at large hadronic recoil, from the light-cone sum rules
(LCSR) with -meson distribution amplitudes in HQET. QCD factorization for
the vacuum-to--meson correlation function with an interpolating current for
the -meson is demonstrated explicitly at one loop with the power counting
scheme . The jet
functions encoding information of the hard-collinear dynamics in the
above-mentioned correlation function are complicated by the appearance of an
additional hard-collinear scale , compared to the counterparts entering
the factorization formula of the vacuum-to--meson correction function for
the construction of from factors. Inspecting the
next-to-leading-logarithmic sum rules for the form factors of indicates that perturbative corrections to the hard-collinear functions
are more profound than that for the hard functions, with the default theory
inputs, in the physical kinematic region. We further compute the subleading
power correction induced by the three-particle quark-gluon distribution
amplitudes of the -meson at tree level employing the background gluon field
approach. The LCSR predictions for the semileptonic form
factors are then extrapolated to the entire kinematic region with the
-series parametrization. Phenomenological implications of our determinations
for the form factors are explored by investigating the
(differential) branching fractions and the ratio of
and by determining the CKM matrix element from the total decay rate
of .Comment: 49 pages, 8 figures, version accepted for publication in JHE
QCD calculations of form factors with higher-twist corrections
We update QCD calculations of form factors at large hadronic
recoil by including the subleading-power corrections from the higher-twist
-meson light-cone distribution amplitudes (LCDAs) up to the twist-six
accuracy and the strange-quark mass effects at leading-power in
from the twist-two -meson LCDA . The higher-twist
corrections from both the two-particle and three-particle -meson LCDAs are
computed from the light-cone QCD sum rules (LCSR) at tree level. In particular,
we construct the local duality model for the twist-five and -six -meson
LCDAs, in agreement with the corresponding asymptotic behaviours at small quark
and gluon momenta, employing the QCD sum rules in heavy quark effective theory
at leading order in . The strange quark mass effects in semileptonic
form factors yield the leading-power contribution in the heavy quark
expansion, consistent with the power-counting analysis in soft-collinear
effective theory, and they are also computed from the LCSR approach due to the
appearance of the rapidity singularities. We further explore the
phenomenological aspects of the semileptonic decays and
the rare exclusive processes , including the determination of
the CKM matrix element , the normalized differential
distributions and precision observables defined by the ratios of branching
fractions for the above-mentioned two channels in the same intervals of .Comment: 36 pages, 9 figure
Chiral selection and frequency response of spiral waves in reaction-diffusion systems under a chiral electric field
Chirality is one of the most fundamental properties of many physical,
chemical and biological systems. However, the mechanisms underlying the onset
and control of chiral symmetry are largely understudied. We investigate
possibility of chirality control in a chemical excitable system (the BZ
reaction) by application of a chiral (rotating) electric field using the
Oregonator model. We find that unlike previous findings, we can achieve the
chirality control not only in the field rotation direction, but also opposite
to it, depending on the field rotation frequency. To unravel the mechanism, we
further develop a comprehensive theory of frequency synchronization based on
the response function approach. We find that this problem can be described by
the Adler equation and show phase-locking phenomena, known as the Arnold
tongue. Our theoretical predictions are in good quantitative agreement with the
numerical simulations and provide a solid basis for chirality control in
excitable media.Comment: 21 pages with 9 figures; update references; to appear in J. Chem.
Phy
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