DIFFERENCES BETWEEN FARMER AND AGENCY ATTITUDES REGARDING POLICIES TO REDUCE PHOSPHORUS POLLUTION IN THE MINNESOTA RIVER BASIN

Environmental Economics and Policy,

ESTIMATING TRANSACTION COSTS OF ALTERNATIVE POLICIES TO REDUCE PHOSPHOROUS POLLUTION IN THE MINNESOTA RIVER

As point sources of pollution reduce their emissions due to water quality control efforts, nonpoint sources have become relatively more important. In the case of agricultural nonpoint source pollution, the policy instruments recommended by economists are not observed in practice. This study was designed to measure the magnitude of transaction costs associated with policies to reduce agricultural nonpoint source pollution and to determine whether transaction costs help explain the prevalence of the policies actually observed. Interviews with staff from governmental agencies were conducted to estimate transaction costs associated with four policies to reduce agricultural phosphorous pollution in the Minnesota River. The tax on phosphate fertilizers had the lowest transaction costs (U.S. $0.94 million), followed by educational programs on best management practices ($3.11 million), the requirement for conservation tillage on all cropped land ($7.85 million), and expansion of a permanent conservation easement program ($9.37 million). Taxes thus may have advantages with respect to transaction costs as well as abatement costs.Environmental Economics and Policy,

Predicting Bleeding and Thrombosis Complications in Patients with Continuous Flow Left Ventricular Assist Devices

Background: Left ventricular assist device (LVAD) therapy has been proven to relieve heart failure symptoms and improve survival, but is not devoid of bleeding and/or thrombotic complications. Risk stratification tools have been utilized in other cardiovascular disease populations to estimate the risk of bleeding and thrombosis with and without anticoagulation, including the HAS-BLED, HEMORR2HAGES, CHADS2 and CHA2DS2-VASc models. The study objective was to evaluate the predictive value of available risk models for bleeding and thrombotic complications in patients with an LVAD within one year of implantation. Methods: This was a retrospective, single-center analysis of patients implanted with the HeartMate II continuous-flow LVAD from July 2011 to June 2016. All patients who received an LVAD within the study period were eligible for inclusion. The primary endpoint was the first occurrence of bleeding or thrombosis within one year from implantation. Baseline risk model scores were calculated at the time of LVAD implantation. Chi-square and student’s t-test were used to measure baseline differences and compare mean risk model scores between patients who had an event. A receiver operator characteristic (ROC) curve analysis was performed to evaluate the accuracy of the risk models to predict an event. Results: A total of 129 patients underwent LVAD implantation within the study time period. Mean CHADS2, CHA2DS2-VASc, and HAS-BLED scores were not significantly different in patients with and without an event. The mean HEMORR2HAGES score was 3.09 and 2.51 in those with and without a bleeding event, respectively (p = 0.008). The ROC curve area for the HEMORR2HAGES model was the highest at 0.620. Conclusion: The HAS-BLED, HEMORR2HAGES, CHADS2and CHA2DS2-VASc risk stratification models did not accurately predict bleeding or thrombosis events in our population. The mean HEMORR2HAGES model score was higher in patients who experienced a bleeding event. However, this model did not have strong positive predictive value. Better risk models are needed to predict bleeding and thrombotic events in this patient population

Stationary probability distributions of stochastic gradient descent and the success and failure of the diffusion approximation

In this thesis, Stochastic Gradient Descent (SGD), an optimization method originally popular due to its computational efficiency, is analyzed using Markov chain methods. We compute both numerically, and in some cases analytically, the stationary probability distributions (invariant measures) for the SGD Markov operator over all step sizes or learning rates. The stationary probability distributions provide insight into how the long-time behavior of SGD samples the objective function minimum. A key focus of this thesis is to provide a systematic study in one dimension comparing the exact SGD stationary distributions to the Fokker-Planck diffusion approximation equations —which are commonly used in the literature to characterize the SGD probability distribution in the limit of small step sizes/learning rates. While various error estimates for the diffusion approximation have recently been established, they are often in a weak sense and not in a strong maximum norm. Our study shows that the diffusion approximation converges with a slow rate in the maximum norm to the true stationary distribution. In addition to large quantitative errors, the exact SGD probability distribution exhibits fundamentally different behavior to the diffusion approximation: they can have compact or singular supports; and there can be multiple invariant measures for non-convex objective functions (when the diffusion approximation only has one). Finally, we use the Markov operator to establish additional results: (1) we show that for quadratic objective functions the SGD expected value is the objective function minimum for any step size. This has the practical implication that time average SGD solutions converge to the minimum even when the SGD iterates never reach or access the minimum. (2) We provide a simple approach to formally derive Fokker-Planck diffusion approximations using only basic calculus (e.g., integration by parts and Taylor expansions), which may be of interest to the engineering community. (3) We observe that the stationary distributions of the Markov operator lead to additional Fokker-Planck equations with simpler diffusion coefficients than what is currently in the literature

Woodrow Wilson as a man of letters

Thesis (M.A.)--Boston Universit

Improving regulatory standards for clearing facilities

Clearinghouses (Banking) ; Payment systems