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J R Baumgardner

Publications and source records attributed to J R Baumgardner.

7 recordsLinked to original sources
PubMed2002

Mantle-circulation models with sequential data assimilation: inferring present-day mantle structure from plate-motion histories.

Data assimilation is an approach to studying geodynamic models consistent simultaneously with observables and the governing equations of mantle flow. Such an approach is essential in mantle circulation models, where we seek to constrain an unknown initial condition some time in the past, and thus cannot hope to use first-principles convection calculations to infer the flow history of the mantle. One of the most important observables for mantle-flow history comes from models of Mesozoic and Cenozoic plate motion that provide constraints not only on the surface velocity of the mantle but also on the evolution of internal mantle-buoyancy forces due to subducted oceanic slabs. Here we present five mantle circulation models with an assimilated plate-motion history spanning the past 120 Myr, a time period for which reliable plate-motion reconstructions are available. All models agree well with upper- and mid-mantle heterogeneity imaged by seismic tomography. A simple standard model of whole-mantle convection, including a factor 40 viscosity increase from the upper to the lower mantle and predominantly internal heat generation, reveals downwellings related to Farallon and Tethys subduction. Adding 35% bottom heating from the core has the predictable effect of producing prominent high-temperature anomalies and a strong thermal boundary layer at the base of the mantle. Significantly delaying mantle flow through the transition zone either by modelling the dynamic effects of an endothermic phase reaction or by including a steep, factor 100, viscosity rise from the upper to the lower mantle results in substantial transition-zone heterogeneity, enhanced by the effects of trench migration implicit in the assimilated plate-motion history. An expected result is the failure to account for heterogeneity structure in the deepest mantle below 1500 km, which is influenced by Jurassic plate motions and thus cannot be modelled from sequential assimilation of plate motion histories limited in age to the Cretaceous. This result implies that sequential assimilation of past plate-motion models is ineffective in studying the temporal evolution of core-mantle-boundary heterogeneity, and that a method for extrapolating present-day information backwards in time is required. For short time periods (of the order of perhaps a few tens of Myr) such a method exists in the form of crude 'backward' convection calculations. For longer time periods (of the order of a mantle overturn), a rigorous approach to extrapolating information back in time exists in the form of iterative nonlinear optimization methods that carry assimilated information into the past through the use of an adjoint mantle convection model.

Computer Simulation↗
PubMed1998

Providing health insurance to the short-term unemployed.

This paper assesses the likely effects of proposed federal programs to provide temporary subsidies to unemployed people for purchasing health insurance. A simulation model, using the Survey of Income and Program Participation and data from other sources, was used to quantify various effects of a typical proposal. The model illustrates how changing eligibility rules and subsidy formulas would alter the cost and other measures of program performance. If the plan were fully operational in 1998, about 1.1 million people (including insured dependents) would get at least a month of new insurance sometime during the year. The program would reduce the number of uninsured by less than half a million people.

Eligibility Determination↗
PubMed1991

Specialization among obstetrician/gynecologists. Another dimension of physician supply.

This study uses the Herfindahl index to investigate the specialization patterns of physicians. "Specialization" is defined as the degree to which an individual doctor concentrates his practice into a narrow range of disease categories. This application is the specialization pattern of office-based obstetrician/gynecologists in the United States. Physician's age and solo practice both exhibit a systematic negative effect on specialization. Both the age and type of practice effects reflect the tendency for younger and nonsolo obstetrician/gynecologists to gear relatively more of their practice into birth-related activities. The age effect is consistent with the hypotheses that either: (1) energy and/or skills for treating pregnancy become more scarce with age, or (2) there is a life-cycle pattern in the doctor/patient relationship with pregnancy serving as an entry point. Malpractice concerns may also contribute to this age effect. The nonsolo practitioner effect suggests the efficiency of multiple doctor practices for allocating physicians' time in activities with high hourly variability in demand. The method used to measure specialization can be extended to investigate other specialty categories and important issues regarding the future supply of physicians' services. These include the effects of an aging physician population and proposed changes in physician reimbursement.

Age Factors↗
PubMed1991

The interaction between forms of insurance contract and types of technical change in medical care.

A simple three-parameter description of medical technology is introduced to investigate the relationships between technical change, welfare, and type of insurance contract. The value of a particular change in technology depends on the existing form of contract. The marginal equilibrium expected utility to consumers of different types of technical change hinges on the manner in which the insurance arrangement is designed to mitigate moral hazard. These results open the way for a positive model of the effects of insurance arrangements on the types of technology that are adopted and the effects of technical changes on the prevalent forms of insurance contract.

Community Participation↗
PubMedDate not supplied

Predicting response to regulatory change in the small group health insurance market: the case of association health plans and HealthMarts.

Lack of health insurance continues to be a concern for many people, even among those who are employed, and employees of small firms are much less likely to be insured than employees of larger firms. For several years, the U.S. Congress has considered legislation that would establish two new vehicles for offering health insurance coverage to small employers: association health plans (AHPs) and HealthMarts. In this paper, we present a model for estimating the impact the new entities would have on coverage and premiums in the small group health insurance market. The model produces a range of estimates based on assumptions, among others, about demand for insurance among small firms and their willingness to switch to less expensive, less generous benefit plans. We estimate that approximately 4.6 million people would obtain coverage through AHPs and HealthMarts, but fewer than half a million of them would be newly insured (based on 1999 population figures). Premiums would increase slightly for firms that continued to purchase coverage in the traditional market.

Community Participation↗
PubMedDate not supplied

Gas chromatographic/mass spectrometric determination of daminozide in high protein food products.

A gas chromatographic/mass spectrometric (GC/MS) method for determining daminozide in high protein products has been developed. Daminozide is hydrolyzed in the presence of a strong base to form unsymmetrical dimethylhydrazine (UDMH) which is then distilled from the food matrix. A stable derivative is formed by reacting UDMH with salicyladehyde to form salicyaldehyde dimethylhydrazone. This derivative is separated and quantitated by GC/MS using selected ion monitoring (SIM) of key ions in the fragmentation pattern: m/z 164 (molecular ion of hydrazone) and m/z 120 (C7H6ON). An internal standard, 4-nitroanisole, is monitored at m/z 153 (molecular ion) and m/z 123 (C6H5O2N). The limit of detection is 0.01 ppm daminozide in a 50 g sample; however, because of variation at low levels, the limit of quantitation is 0.1 ppm. Recoveries are 90% or greater from peanuts and peanut butter spiked at the 0.1-2 ppm level. Reproducibility of the method depends on the food matrix and is 26% RSD in the worst case. Data are compared for the GC/MS method and the official EPA colorimetric procedure. Results showed a high bias in the colorimetric method, especially when roasted peanut products were analyzed.

Arachis↗