Browsing by Author "Zhang, J"
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Item Open Access Hospital avoidance and unintended deaths during the covid-19 pandemic(American Journal of Health Economics, 2021-09-01) Zhang, JThe COVID-19 pandemic significantly altered individual behaviors, including the consumption of health care. I study utilization and mortality in the largest integrated healthcare system in the United States, the Veterans Health Administration, and find that between the middle of March and the beginning of May 2020, emergency department and inpatient hospital visits declined by 37 percent and 46 percent, and remained 10 percent and 17 percent below expected levels by the end of October. Declines were more pronounced for non-urgent and non-life-threatening conditions, although urgent and life-threatening conditions also dropped by a quarter during the early months. Conditional on arrival at the emergency department, conditions were more severe at presentation. In the first two months of the pandemic, veteran mortality increased by 19.5 percent, yet non-COVID-19 mortality in VA inpatient settings declined. I find suggestive evidence that hospital avoidance may have resulted in higher non-COVID-19 mortality. By focusing on counties with no official COVID-19 deaths by May 19, 2020, I estimate that an upper bound of 7.9 percent of excess veteran deaths in the first two months of the pandemic were due to hospital avoidance.Item Open Access Jamming for a 2D granular material(Soft Matter, 2010-07-07) Zhang, J; Majmudar, TS; Sperl, M; Behringer, RPThis paper focuses on the nature of jamming, as seen in two-dimensional frictional granular systems consisting of photoelastic particles. The photoelastic technique is unique at this time, in its capability to provide detailed particle-scale information on forces and kinematic quantities such as particle displacements and rotations. These experiments first explore isotropic stress states near point J through measurements of the mean contact number per particle, Z, and the pressure, P as functions of the packing fraction, . In this case, the experiments show some but not all aspects of jamming, as expected on the basis of simulations and models that typically assume conservative, hence frictionless, forces between particles. Specifically, there is a rapid growth in Z, at a reasonable which we identify with as c. It is possible to fit Z and P, to power law expressions in - c above c, and to obtain exponents that are in agreement with simulations and models. However, the experiments differ from theory on several points, as typified by the rounding that is observed in Z and P near c. The application of shear to these same 2D granular systems leads to phenomena that are qualitatively different from the standard picture of jamming. In particular, there is a range of packing fractions below c, where the application of shear strain at constant leads to jammed stress-anisotropic states, i.e. they have a non-zero shear stress, τ. The application of shear strain to an initially isotropically compressed (hence jammed) state, does not lead to an unjammed state per se. Rather, shear strain at constant first leads to an increase of both τ and P. Additional strain leads to a succession of jammed states interspersed with relatively localized failures of the force network leading to other stress-anisotropic states that are jammed at typically somewhat lower stress. The locus of jammed states requires a state space that involves not only and τ, but also P. P, τ, and Z are all hysteretic functions of shear strain for fixed . However, we find that both P and τ are roughly linear functions of Z for strains large enough to jam the system. This implies that these shear-jammed states satisfy a Coulomb like-relation, τ = μP. © 2010 The Royal Society of Chemistry.Item Open Access Magnetic superlens-enhanced inductive coupling for wireless power transfer(Journal of Applied Physics, 2012-03-15) Huang, D; Urzhumov, Y; Smith, DR; Hoo Teo, K; Zhang, JWe investigate numerically the use of a negative-permeability perfect lens for enhancing wireless power transfer between two current carrying coils. The negative permeability slab serves to focus the flux generated in the source coil to the receiver coil, thereby increasing the mutual inductive coupling between the coils. The numerical model is compared with an analytical theory that treats the coils as point dipoles separated by an infinite planar layer of magnetic material Urzhumov, Phys. Rev. B 19, 8312 (2011). In the limit of vanishingly small radius of the coils, and large width of the metamaterial slab, the numerical simulations are in excellent agreement with the analytical model. Both the idealized analytical and realistic numerical models predict similar trends with respect to metamaterial loss and anisotropy. Applying the numerical models, we further analyze the impact of finite coil size and finite width of the slab. We find that, even for these less idealized geometries, the presence of the magnetic slab greatly enhances the coupling between the two coils, including cases where significant loss is present in the slab. We therefore conclude that the integration of a metamaterial slab into a wireless power transfer system holds promise for increasing the overall system performance. © 2012 American Institute of Physics.Item Open Access Pathways into Opioid Dependence: Evidence from Practice Variation in Emergency Departments(American Economic Journal: Applied Economics, 2022-01-01) Eichmeyer, S; Zhang, JWe use practice variation across physicians to uncover the role of medical care in causing opioid dependence. Using health records of 2 million US veterans with emergency department visits, we find that quasi-random assignment to a top (versus bottom) decile prescribing provider significantly increases subsequent opioid use and misuse rates. Instrumental variable results show that opioid prescription receipt leads to a 20 percent increase in the probability of long-term prescription opioid use and sizable increases in the development of opioid use disorder and opioid overdose mortality. We find suggestive evidence of transition into illicit opioids due to prescription opioid exposure.Item Open Access Primary care providers’ influence on opioid use and its adverse consequences(Journal of Public Economics, 2023-01-01) Eichmeyer, S; Zhang, JPrimary care is the most frequently utilized health service and is the source of nearly half of all opioids prescribed in the United States. This paper studies the impact of exposure to high prescribing primary care providers (PCP) on opioid use, opioid use disorder, and mental health. Using electronic health records, we exploit within-facility quasi-random assignment of providers, who differ in their opioid prescribing tendency, to 650,000 new patients enrolling in care with the Veterans Health Administration. We find that assignment to a PCP who prescribes opioids at a 2.54 percentage point (pp) higher rate (equivalent to the average difference between a 90th and 10th percentile prescriber within a facility) increases the probability of long-term opioid use by 20% (or 0.43 pp) and development of opioid use disorder by 4% (or 0.035 pp). Veterans’ mental health deteriorates: the three-year likelihood of a depression diagnosis increases by 1.3% (or 0.31 pp). We find that PCPs with more cautious prescribing behavior rely more on non-opioid pain management and adhere more to clinical recommendations on naloxone distribution.Item Open Access Relationship between giant panda populations and selected ecosystem services(Ecosystem Services, 2020-08-01) Zhang, J; Pimm, SL; Xu, W; Shi, X; Xiao, Y; Kong, L; Fan, X; Ouyang, ZThe Convention on Biological Diversity's Aichi Targets address both biodiversity and ecosystem services. We explore the relationship between giant panda populations and three ecosystem services: carbon sequestration, water retention, and soil retention. Do pandas prefer areas with higher than average values of these services? Areas may be good for pandas but not for these ecosystem services, and vice versa. Answering these questions can focus panda conservation. We map their spatial distribution and temporal changes from 2000 to 2015, by watershed, to target future protected areas for both pandas and these ecosystem services. Pandas occupy watersheds with above-average carbon sequestration and water retention. There is no tendency for pandas to be increasing in watersheds that have higher than average values of these ecosystem services or in watersheds where they are improving. Protected areas represented watersheds with higher than average values of these ecosystem services but without pandas only poorly. Watersheds with pandas do provide higher than average ecosystem services, but watersheds above average for these ecosystem services often lack pandas. Those areas might be potentially important for pandas, but obstacles block their way. We identified conservation areas combining habitats, population, activity range, and higher than average values of these ecosystem services and then proposed new protected areas.Item Open Access Serum cholesterol levels within the high normal range are associated with better cognitive performance among Chinese elderly(Journal of Nutrition, Health and Aging, 2016-03-01) Lv, Y-B; Yin, ZX; Chei, C-L; Brasher, MS; Zhang, J; Kraus, VB; Qian, F; Shi, X-M; Matchar, DB; Zeng, Y© 2016, Serdi and Springer-Verlag France.Objectives: The association between cognitive function and cholesterol levels is poorly understood and inconsistent results exist among the elderly. The purpose of this study is to investigate the association of cholesterol level with cognitive performance among Chinese elderly. Design: A cross-sectional study was implemented in 2012 and data were analyzed using generalized additive models, linear regression models and logistic regression models. Setting: Community-based setting in eight longevity areas in China. Subjects: A total of 2000 elderly aged 65 years and over (mean 85.8±12.0 years) participated in this study. Measurements: Total cholesterol (TC), triglycerides (TG), low density lipoprotein cholesterol (LDL-C) and high density lipoprotein cholesterol (HDL-C) concentration were determined and cognitive impairment was defined as Mini-Mental State Examination (MMSE) score≤23. Results: There was a significant positive linear association between TC, TG, LDL-C, HDL-C and MMSE score in linear regression models. Each 1 mmol/L increase in TC, TG, LDL-C and HDL-C corresponded to a decreased risk of cognitive impairment in logistic regression models. Compared with the lowest tertile, the highest tertile of TC, LDL-C and HDL-C had a lower risk of cognitive impairment. The adjusted odds ratios and 95% CI were 0.73(0.62–0.84) for TC, 0.81(0.70–0.94) for LDL-C and 0.81(0.70–0.94) for HDL-C. There was no gender difference in the protective effects of high TC and LDL-C levels on cognitive impairment. However, for high HDL-C levels the effect was only observed in women. High TC, LDL-C and HDL-C levels were associated with lower risk of cognitive impairment in the oldest old (aged 80 and older), but not in the younger elderly (aged 65 to 79 years). Conclusions: These findings suggest that cholesterol levels within the high normal range are associated with better cognitive performance in Chinese elderly, specifically in the oldest old. With further validation, low cholesterol may serve a clinical indicator of risk for cognitive impairment in the elderly.Item Open Access Serum cholesterol levels within the high normal range are associated with better cognitive performance among Chinese elderly(Journal of Nutrition, Health and Aging, 2016-03-01) Lv, YB; Yin, ZX; Chei, CL; Brasher, MS; Zhang, J; Kraus, VB; Qian, F; Shi, X; Matchar, DB; Zeng, YObjectives: The association between cognitive function and cholesterol levels is poorly understood and inconsistent results exist among the elderly. The purpose of this study is to investigate the association of cholesterol level with cognitive performance among Chinese elderly. Design: A cross-sectional study was implemented in 2012 and data were analyzed using generalized additive models, linear regression models and logistic regression models. Setting: Community-based setting in eight longevity areas in China. Subjects: A total of 2000 elderly aged 65 years and over (mean 85.8±12.0 years) participated in this study. Measurements: Total cholesterol (TC), triglycerides (TG), low density lipoprotein cholesterol (LDL-C) and high density lipoprotein cholesterol (HDL-C) concentration were determined and cognitive impairment was defined as Mini-Mental State Examination (MMSE) score≤23. Results: There was a significant positive linear association between TC, TG, LDL-C, HDL-C and MMSE score in linear regression models. Each 1 mmol/L increase in TC, TG, LDL-C and HDL-C corresponded to a decreased risk of cognitive impairment in logistic regression models. Compared with the lowest tertile, the highest tertile of TC, LDL-C and HDL-C had a lower risk of cognitive impairment. The adjusted odds ratios and 95% CI were 0.73(0.62–0.84) for TC, 0.81(0.70–0.94) for LDL-C and 0.81(0.70–0.94) for HDL-C. There was no gender difference in the protective effects of high TC and LDL-C levels on cognitive impairment. However, for high HDL-C levels the effect was only observed in women. High TC, LDL-C and HDL-C levels were associated with lower risk of cognitive impairment in the oldest old (aged 80 and older), but not in the younger elderly (aged 65 to 79 years). Conclusions: These findings suggest that cholesterol levels within the high normal range are associated with better cognitive performance in Chinese elderly, specifically in the oldest old. With further validation, low cholesterol may serve a clinical indicator of risk for cognitive impairment in the elderly.Item Open Access Stress correlations in granular materials: An entropic formulation(Physical Review E - Statistical, Nonlinear, and Soft Matter Physics, 2009-12-28) Lois, G; Zhang, J; Majmudar, TS; Henkes, S; Chakraborty, B; O'Hern, CS; Behringer, RPWe study the response of dry granular materials to external stress using experiment, simulation, and theory. We derive a Ginzburg-Landau functional that enforces mechanical stability and positivity of contact forces. In this framework, the elastic moduli depend only on the applied stress. A combination of this feature and the positivity constraint leads to stress correlations whose shape and magnitude are extremely sensitive to the nature of the applied stress. The predictions from the theory describe the stress correlations for both simulations and experiments semiquantitatively. © 2009 The American Physical Society.Item Open Access Structural templates of disordered granular media(International Journal of Solids and Structures, 2015-01-01) Walker, DM; Tordesillas, A; Zhang, J; Behringer, RP; Andò, E; Viggiani, G; Druckrey, A; Alshibli, K© 2014 Elsevier Ltd. All rights reserved.Granular materials, in common with many complex systems, exhibit a range of self-organization processes that control their mechanical performance. Many of these processes directly manifest in the evolution of the contact network as the material responds to applied stresses and strains. Yet the connections between the topology, structure and dynamics of this evolving contact network remain poorly understood. Here we demonstrate that dense granular systems under a variety of loading conditions exhibit preferred structural ordering reminiscent of a superfamily classification. In particular, two distinct superfamilies are discovered: the first is typically exhibited by materials in the pre-failure regime, while the second manifests in the unstable or failure regime. We demonstrate the robustness of these findings with respect to a range of packing fractions in experimental sand and photoelastic disk assemblies subject to compression and shear, as well as in a series of discrete element simulations of compression tests. We show that the superfamily classification of small connected subgraphs in a granular material can be used to map boundaries in a so-called jamming phase diagram and, consequently, offers a key opportunity to bridge the mechanics and physics perspectives on the constitutive behavior of granular systems.