Doubly robust estimation and sensitivity analysis for marginal structural quantile models

The marginal structure quantile model (MSQM) is a useful tool to characterize the causal effect of a time-varying treatment on the full distribution of potential outcomes. However, to date, only the inverse probability weighting (IPW) approach has been developed to identify the structural causal parameters in MSQM, which requires correct specification of the propensity score models for the treatment assignment mechanism. We propose a doubly robust approach for the MSQM under the semiparametric framework. We derive the efficient influence function associated with a MSQM and estimate causal parameters in the MSQM by combining IPW and a new iterative conditional regression approach that models the full potential outcome distribution. The proposed approach is consistent if either of the models associated with treatment assignment or the potential outcome distributions is correctly specified, and is locally efficient if both models are correct. To implement the doubly robust MSQM estimator, we propose to solve a smoothed estimating equation to facilitate efficient computation of the point and variance estimates. In addition, we develop a new confounding function and sensitivity analysis strategy to investigate the robustness of several MSQM estimators when the no unmeasured confounding assumption is violated. We apply the proposed methods to the Yale New Haven Health System Electronic Health Record data to study the causal effect of antihypertensive medications to inpatients with severe hypertension, and assess the robustness of findings to unmeasured time-varying confounding

Performance Analysis of the Unary Coding Aided SWIPT in a Single-User Z-Channel

Radio frequency (RF) signal based simultaneous wireless information and power transfer (SWIPT) has emerged as a promising technique for satisfying both the communication and charging requests of the massively deployed IoT devices. Different from the physical layer and the medium-access-control layer design for coordinating the SWIPT in the RF band, we study its coding-level control from the information theoretical perspective. Due to its practical implementation of the decoder and its flexibility on the codeword structure, the unary code is chosen as a potential joint information and energy encoder. By conceiving the classic Z-channel, the mutual information and the energy harvesting performance of the unary coding aided SWIPT transceiver is analysed. Furthermore, the optimal codeword distribution is obtained for maximising the mutual information, while satisfying the minimum energy harvesting requirement. Our theoretical analysis and the optimal coding design are demonstrated by the numerical results

Association of chemokine receptor gene (CCR2-CCR5) haplotypes with acquisition and control of HIV-1 infection in Zambians

<p>Abstract</p> <p>Background</p> <p>Polymorphisms in chemokine (C-C motif) receptors 2 and 5 genes (<it>CCR2 </it>and <it>CCR5</it>) have been associated with HIV-1 infection and disease progression. We investigated the impact of <it>CCR2-CCR5 </it>haplotypes on HIV-1 viral load (VL) and heterosexual transmission in an African cohort. Between 1995 and 2006, cohabiting Zambian couples discordant for HIV-1 (index seropositive and HIV-1 exposed seronegative {HESN}) were monitored prospectively to determine the role of host genetic factors in HIV-1 control and heterosexual transmission. Genotyping for eight <it>CCR2 </it>and <it>CCR5 </it>variants resolved nine previously recognized haplotypes. By regression and survival analytic techniques, controlling for non-genetic factors, we estimated the effects of these haplotypic variants on a) index partner VL, b) seroconverter VL, c) HIV-1 transmission by index partners, d) HIV-1 acquisition by HESN partners.</p> <p>Results</p> <p>Among 567 couples, 240 virologically linked transmission events had occurred through 2006. HHF*2 homozygosity was associated with significantly lower VL in seroconverters (mean beta = -0.58, log₁₀<it>P </it>= 0.027) and the HHD/HHE diplotype was associated with significantly higher VL in the seroconverters (mean beta = 0.54, log₁₀<it>P </it>= 0.014) adjusted for age and gender in multivariable model. HHD/HHE was associated with more rapid acquisition of infection by the HESNs (HR = 2.0, 95% CI = 1.20-3.43, <it>P </it>= 0.008), after adjustments for index partner VL and the presence of genital ulcer or inflammation in either partner in Cox multivariable models. The HHD/HHE effect was stronger in exposed females (HR = 2.1, 95% CI = 1.14-3.95, <it>P </it>= 0.018).</p> <p>Conclusions</p> <p>Among Zambian discordant couples, HIV-1 coreceptor gene haplotypes and diplotypes appear to modulate HIV-1 VL in seroconverters and alter the rate of HIV-1 acquisition by HESNs. These associations replicate or resemble findings reported in other African and European populations.</p