Harmonically Confined Particles with Long-Range Repulsive Interactions.

We study an interacting system of N classical particles on a line at thermal equilibrium. The particles are confined by a harmonic trap and repel each other via pairwise interaction potential that behaves as a power law ∝∑[under i≠j][over N]|x_{i}-x_{j}|^{-k} (with k>-2) of their mutual distance. This is a generalization of the well-known cases of the one-component plasma (k=-1), Dyson's log gas (k→0^{+}), and the Calogero-Moser model (k=2). Because of the competition between harmonic confinement and pairwise repulsion, the particles spread over a finite region of space for all k>-2. We compute exactly the average density profile for large N for all k>-2 and show that while it is independent of temperature for sufficiently low temperature, it has a rich and nontrivial dependence on k with distinct behavior for -21 and k=1.

Width Scaling of an Interface Constrained by a Membrane.

We investigate the shape of a growing interface in the presence of an impenetrable moving membrane. The two distinct geometrical arrangements of the interface and membrane, obtained by placing the membrane behind or ahead of the interface, are not symmetrically related. On the basis of numerical results and an exact calculation, we argue that these two arrangements represent two distinct universality classes for interfacial growth: while the well-established Kardar-Parisi-Zhang (KPZ) growth is obtained in the "ahead" arrangement, we find an arrested KPZ growth with a smaller roughness exponent in the "behind" arrangement. This suggests that the surface properties of growing cell membranes and expanding bacterial colonies, for example, are fundamentally distinct.

Phase diagram and density large deviations of a nonconserving ABC model.

The effect of particle-nonconserving processes on the steady state of driven diffusive systems is studied within the context of a generalized ABC model. It is shown that in the limit of slow nonconserving processes, the large deviation function of the overall particle density can be computed by making use of the steady-state density profile of the conserving model. In this limit one can define a chemical potential and identify first order transitions via Maxwell's construction, similarly to what is done in equilibrium systems. This method may be applied to other driven models subjected to slow nonconserving dynamics.

Long-range correlations and ensemble inequivalence in a generalized ABC model.

A generalization of the ABC model, a one-dimensional model of a driven system of three particle species with local dynamics, is introduced, in which the model evolves under either (i) density-conserving or (ii) nonconserving dynamics. For equal average densities of the three species, both dynamical models are demonstrated to exhibit detailed balance with respect to a Hamiltonian with long-range interactions. The model is found to exhibit two distinct phase diagrams, corresponding to the canonical (density-conserving) and grand canonical (density nonconserving) ensembles, as expected in long-range interacting systems. The implications of this result to nonequilibrium steady states, such as those of the ABC model with unequal average densities, are briefly discussed.

Supercoil formation in DNA denaturation.

We generalize the Poland-Scheraga model to the case of a circular DNA, taking into account the twisting of the two strains around each other. Guided by recent single-molecule experiments on DNA strands, we assume that the torsional stress induced by denaturation enforces the formation of supercoils whose writhe absorbs the linking number expelled by the loops. Our model predicts that when the entropy parameter of a loop satisfies c2, a first-order denaturation transition is consistent with our model and may take place in the actual system, as in the case with no supercoils. These results are in contrast with other treatments of circular DNA melting where denaturation is assumed to be accompanied by an increase in twist rather than writhe on the bound segments.

Dynamics of DNA melting.

The dynamics of loops at the DNA denaturation transition is studied. A scaling argument is used to evaluate the asymptotic behavior of the autocorrelation function of the state of complementary bases (either open or closed). The long-time asymptotic behavior of the autocorrelation function is expressed in terms of the entropy exponent, c, of a loop. The validity of the scaling argument is tested using a microscopic model of an isolated loop and a toy model of interacting loops. This suggests a method for measuring the entropy exponent using single-molecule experiments such as fluorescence correlation spectroscopy.

Driven tracers in narrow channels.

Steady-state properties of a driven tracer moving in a narrow two-dimensional (2D) channel of quiescent medium are studied. The tracer drives the system out of equilibrium, perturbs the density and pressure fields, and gives the bath particles a nonzero average velocity, creating a current in the channel. Three models in which the confining effect of the channel is probed are analyzed and compared in this study: the first is the simple symmetric exclusion process (SSEP), for which the stationary density profile and the pressure on the walls in the frame of the tracer are computed. We show that the tracer acts like a dipolar source in an average velocity field. The spatial structure of this 2D strip is then simplified to a one-dimensional (1D) SSEP, in which exchanges of position between the tracer and the bath particles are allowed. Using a combination of mean-field theory and exact solution in the limit where no exchange is allowed gives good predictions of the velocity of the tracer and the density field. Finally, we show that results obtained for the 1D SSEP with exchanges also apply to a gas of overdamped hard disks in a narrow channel. The correspondence between the parameters of the SSEP and of the gas of hard disks is systematic and follows from simple intuitive arguments. Our analytical results are checked numerically.

Critical phase in nonconserving zero-range processes and rewiring networks.

Zero-range processes, in which particles hop between sites on a lattice, are closely related to rewiring networks, in which rewiring of links between nodes takes place. Both systems exhibit a condensation transition for appropriate choices of the dynamical rules. The transition results in a macroscopically occupied site for zero-range processes and a macroscopically connected node for networks. Criticality, characterized by a scale-free distribution, is obtained only at the transition point. This is in contrast with the widespread scale-free complex networks. Here we propose a generalization of these models whereby criticality is obtained throughout an entire phase, and the scale-free distribution does not depend on any fine-tuned parameter.

Directed percolation with long-range interactions: Modeling nonequilibrium wetting.

It is argued that some phase transitions observed in models of nonequilibrium wetting phenomena are related to contact processes with long-range interactions. This is investigated by introducing a model where the activation rate of a site at the edge of an inactive island of length l is 1+a l(-sigma) . Mean-field analysis and numerical simulations indicate that for sigma>1 the transition is continuous and belongs to the universality class of directed percolation, while for 0

Cost utility of public clinics to increase pneumococcal vaccines in the elderly.

BACKGROUND: Pneumococcal immunization has been shown to be cost effective, is recommended by the Advisory Committee on Immunization Practices, and is covered by Medicare. Despite that, over 50% of the population aged > or =65 is not vaccinated, leading to significant mortality and morbidity. The objective of this study is to evaluate the costs and the cost utility of immunization in nontraditional settings (community clinics set up to provide influenza and pneumococcal vaccinations) as a strategy to increase pneumococcal immunization rates. METHODS: A cost-utility analysis of public immunization clinics in Monroe County, New York, during the fall of 1998. The study included 1207 adults aged > or =65. Costs of operating the clinics and of vaccine administration were measured. The cost of health sequela and estimates of quality-adjusted life years (QALYs) were obtained from prior studies. Sensitivity analyses were performed to test several important assumptions. RESULTS: Unlike immunizations in physician offices, immunizations in nontraditional settings are not cost saving. Estimates of incremental cost-utility ratios ranged from $4215 per QALY to $12,617 per QALY, depending on the underlying assumptions of the model. CONCLUSIONS: Clinics in nontraditional settings offering pneumococcal immunization have cost-utility ratios near and below those of other recommended vaccines. These results suggest that such clinics should be considered a viable strategy for increasing pneumococcal immunization rates.

HMO penetration, competition, and risk-adjusted hospital mortality.

OBJECTIVE: HMOs have been shown to have an effect on the care provided directly to their enrollees. They may also influence the care provided to individuals in fee-for-service plans through a spill-over effect. The objective of this study was to investigate the associations among HMO market penetration, HMO and hospital competition, and the quality of care received by Medicare fee-for-service patients measured by risk-adjusted hospital mortality rates. DATA SOURCES: The 1990 data for 1,927 hospitals in 134 metropolitan statistical areas (with five or more hospitals) included Medicare fee-for-service risk-adjusted mortality rates from the Medicare Hospital Information Reports, hospital characteristics from the American Hospital Association annual survey, and HMO market penetration and competition calculated from InterStudy and Group Health Association of America data. STUDY DESIGN: Statistical regression techniques were used to identify the associations between HMO market penetration, competition, and risk-adjusted mortality, controlling for other hospital characteristics and region. PRINCIPAL FINDINGS: Higher HMO market penetration and to a lesser degree increased HMO competition were associated with better mortality outcomes for fee-for-service Medicare enrollees. Competition between hospitals did not exhibit a significant association. CONCLUSIONS: HMOs may have a spill-over effect on quality of care received by individuals enrolled in fee-for-service plans. These findings may be explained by a positive effect on local practice styles or a preferential selection by HMOs for areas with better hospital care.

Do quality report cards play a role in HMOs' contracting practices? Evidence from New York State.

OBJECTIVE: To answer two related questions: (1) Do managed care organizations (MCOs) in New York State (NYS) consider quality when they choose cardiac surgeons? (2) Do they use information about risk-adjusted mortality rates (RAMR) provided in the New York State Cardiac Surgery Reports? DATA SOURCES: (1) Telephone interviews with and contracting data from the majority of MCOs licensed in NYS; (2) RAMR, quality outlier designation, and procedure volume for all cardiac surgeons, as reported in the Cardiac Surgery Reports. STUDY DESIGN: Interview data were analyzed in conjunction with patterns revealed by contracting data. Null hypotheses that MCOs' contracting choices were random with respect to the information published in the Cardiac Surgery Reports were tested. PRINCIPAL FINDINGS: Sixty percent of MCOs ranked the quality of surgeons as most important in their contracting considerations. Although 64 percent of MCOs indicated some knowledge of the NYS Cardiac Surgery Reports, only 20 percent indicated that the reports were a major factor in their contracting decision. Analyses of actual contracting patterns show that in aggregate, the hypothesis of random choice could be rejected with respect to high-quality outlier status and high procedure volume but not for RAMR or poor-quality outlier status. The panel composition of the majority of MCOs (80.2 percent) was within two standard deviations of the expected mean under the null hypothesis. CONCLUSIONS: Despite a professed preference for high-quality surgeons, the use of publicly available quality reports by MCOs is currently low, and contracting practices for the majority of MCOs do not indicate a systematic selection either for or against surgeons based on their reported mortality scores. This study suggests that policy initiatives to increase the effective use of report cards should be encouraged.

Barriers to compliance with screening guidelines for diabetic retinopathy.

OBJECTIVE: To identify barriers to compliance with guidelines for diabetic retinopathy screening. METHODS: The population studied included 4410 adults, aged 31 to 64, enrolled in an Independent Practice Association (IPA) plan in Upstate New York, who were diagnosed with diabetes, and their Primary Care Physicians (408 PCPs). Claims data were used to calculate variables characterizing patients and their PCPs. Logistic regression models were estimated to identify factors associated with higher probability of screening. RESULTS: 34% of patients were screened in 1993. The probability of screening was significantly higher for older patients, for women, for patients who visit their PCPs more often and for those living in areas of higher average education and lower percentage of blacks. However, only 16% of diabetic patients received an annual screen in two consecutive years (1992 and 1993). The probability of consecutive annual screening was significantly associated only with gender and patient expenditures per month. CONCLUSION: The very low rate of diabetic retinopathy screening has implications for quality of life of patients with diabetes, long term costs of caring for them and social costs due to lost productivity. Interventions to increase screening rates are needed and should target both patients and their Primary Care Physicians.

Stability of disability among PACE enrollees: financial and programmatic implications.

This article examines the experience of the first 11 Program of All-inclusive Care for the Elderly (PACE) programs. It investigates changes in functional status of participants in relation to length of enrollment in the program and individual risk characteristics. Our findings indicate that mature programs experience stable disability mix over time, supporting the rationale for the current PACE payment method. However, significant differences exist between programs, suggesting that payment rates could be more program specific. Analysis of the effect of patient characteristics at admission on the likelihood of improvement in functional status identified areas for quality improvement. The implications of this study have increasing importance in light of the expected expansion of PACE to approximately 100 sites by the year 2000.

The influence of risk adjustment methods on conclusions about quality of care in nursing homes based on outcome measures.

This study compares quality rankings of about 550 nursing homes in New York State, based on several outcome measures and differing risk adjustment methods. The outcomes were decline in functional status, worsening decubiti, and prevalence of physical restraints. Measures were constructed from PRI data, which are similar to MDS data. We found substantial disagreement on quality ranking across measures due to differences in the scope of risk adjustment. Insufficient risk adjustment of outcome measures may, therefore, lead to inappropriate classification of nursing homes as either poor-quality or high-quality homes. This has implications for state quality oversight, providers' reputations and patients' choice.

The effect of accurate patient screening on the cost-effectiveness of case management programs.

Case management programs are expensive and therefore require careful screening of enrollees to ensure cost-effectiveness. Screening tools, however, are imperfect, with positive predictive values usually below 50%. This article examines the relationship between the accuracy of the screening tools and the cost-effectiveness of case management. Using data from a Medicare health maintenance organization (HMO), we develop an optimized 5-question screening tool. We then simulate the use of this screening tool and its impact on the cost-effectiveness of several hypothetical case management programs. The article demonstrates that even screening tools with only 20-30% positive predictive value could turn a case management program into a cost-effective program.

Ordering dynamics of the driven lattice-gas model.

The evolution of a two-dimensional driven lattice-gas model is studied on an LxxL(y) lattice. Scaling arguments and extensive numerical simulations are used to show that starting from random initial configuration the model evolves via two stages: (a) an early stage in which alternating stripes of particles and vacancies are formed along the direction y of the driving field, and (b) a stripe coarsening stage, in which the number of stripes is reduced and their average width increases. The number of stripes formed at the end of the first stage is shown to be a function of L(x)/L(straight phi)(y), with straight phi approximately 0.2. Thus, depending on this parameter, the resulting state could be either single or multistriped. In the second, stripe coarsening stage, the coarsening time is found to be proportional to L(y), becoming infinitely long in the thermodynamic limit. This implies that the multistriped state is thermodynamically stable. The results put previous studies of the model in a more general framework.

Wetting under nonequilibrium conditions.

We report a detailed account of the phase diagram of a recently introduced model for nonequilibrium wetting in (1+1) dimensions [H. Hinrichsen, R. Livi, D. Mukamel, and A. Politi, Phys. Rev. Lett. 79, 2710 (1997)]. A mean-field approximation is shown to reproduce the main features of the phase diagram, while providing indications for the behavior of the wetting transition in higher dimensions. The mean-field phase diagram is found to exhibit an extra transition line which does not exist in (1+1) dimensions. The line separates a phase in which the interface height distribution decays exponentially at large heights from a superexponentially decaying phase. Implications to wetting in dimensions higher than (1+1) are discussed.

Hard disks in narrow channels.

The thermodynamic and dynamical behavior of a gas of hard disks in a narrow channel is studied theoretically and numerically. Using a virial expansion, we find that the pressure and collision frequency curves exhibit a singularity at a channel width corresponding to twice the disk diameter. As expected, the maximum Lyapunov exponent is also found to display a similar behavior. At high density, these curves are dominated by solidlike configurations which are different from the bulk ones, due to the channel boundary conditions.