42,016 research outputs found

    Influence of Water Depth on the Rate of Expansion of Giant Cutgrass Populations and Management Implications

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    Giant cutgrass ( Zizaniopsis miliacea (Michx.) Doell. & Asch.), a tall emergent grass native to the southeastern United States, was studied in Lake Seminole where it formed large expanding stands, and Lake Alice where it was confined to a stable narrow fringe

    Measuring the Efficiency of an FCC Spectrum Auction

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    FCC spectrum auctions sell licenses to provide mobile phone service in designated geographic territories. We propose a method to structurally estimate the deterministic component of bidder valuations and apply it to the 1995–1996 C-block auction. We base our estimation of bidder values on a pairwise stability condition, which implies that two bidders cannot exchange licenses in a way that increases total surplus. Pairwise stability holds in many theoretical models of simultaneous ascending auctions, including some models of intimidatory collusion and demand reduction. Pairwise stability is also approximately satisfied in data that we examine from economic experiments. The lack of post-auction resale also suggests pairwise stability. Using our estimates of deterministic valuations, we measure the allocative efficiency of the C-block outcome.

    Multistage depressed collector with efficiency of 90 to 94 percent for operation of a dual-mode traveling wave tube in the linear region

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    An axisymmetric, multistage, depressed collector of fixed geometric design was evaluated in conjunction with an octave bandwidth, dual mode traveling wave tube (TWT). The TWT was operated over a wide range of conditions to simulate different applications. The collector performance was optimized (within the constraint of fixed geometric design) over the range of TWT operating conditions covered. For operation of the TWT in the linear, low distortion range, 90 percent and greater collector efficiencies were obtained leading to TWT overall efficiencies of 20 to 35 percent, as compared with 2 to 5 percent with an undepressed collector. With collectors of this efficiency and minimized beam interception losses, it becomes practical to design dual mode TWT's such that the low mode can represent operation well below saturation. Consequently, the required pulse up in beam current can be reduced or eliminated, and this mitigates beam control and dual mode TWT circuit design problems. For operation of the dual mode TWT at saturation, average collector efficiencies in excess of 85 percent were obtained for both the low and high modes across an octave bandwidth, leading to a three to fourfold increase in the TWT overall efficiency

    Home-Based Therapy for Young Children in Low-Income Families: A Student Training Program

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    The purpose of this project was to develop an internship training program that offered in-home therapy for young children with significant emotional and behavior problems. The children lived in single-parent, low-income homes in unsafe neighborhoods of a large, urban area. A year-long, training and supervision program was implemented with 10 second-year, graduate students enrolled in 5 different university programs that prepared mental health professionals. Students received specialized instruction in working with diverse families living in poverty and in an evidence-based treatment program. They initially observed veteran counselors implementing the treatment program in homes and gradually assumed responsibility for conducting sessions on their own. Students’ scores on a measure of counseling self-efficacy improved significantly from pre-to post-internship. Students reported high levels of satisfaction with the training program and significantly improved confidence levels in their counseling skills at the conclusion of their internship experience. The limitations of these preliminary outcomes for this pilot program are discussed along with the need for more research in this understudied area

    Efficiency enhancement of dual-mode traveling wave tubes at saturation and in the linear range by use of spent-beam refocusing and multistage depressed collectors

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    An axisymmetric, multistage depressed collector of fixed geometric design was evaluated in conjunction with an octave-bandwidth, dual-mode TWT. The TWT was operated over a wide range of conditions to simulate different applications. The collector was operated in three-, four-, and five-stage configurations, and its performance was optimized (within the constraint of fixed geometric design) over the range of TWT operating conditions covered. For operation of the dual-mode TWT at and near saturation, the collectors increased the TWT overall efficiency by a factor of 2 1/2 to 3 1/2. Collector performance was relatively constant for both the high and low TWT modes and for operation of the TWT across an octave bandwidth. For operation of the TWT in the linear, low-distortion range, collector efficiencies of 90 percent and greater were obtained, leading to a five- to twelvefold increase in the TWT overall efficiency for the range of operating conditions covered and reasonably high (greater than 25 percent) overall efficiencies well below saturation

    Do Input Quality and Structural Productivity Estimates Drive Measured Differences in Firm Productivity?

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    Firms in the same industry can differ in measured total factor productivity (TFP) by multiples of 3. Griliches (1957) suggests one explanation: the quality of inputs differs across firms. Labor inputs are traditionally measured only as the number of workers. We investigate whether adjusting for the quality of labor inputs substantially decreases measured TFP dispersion. We add labor market history variables such as experience and firm and industry tenure, as well as general human capital measures such as schooling and sex. We also investigate whether an innovative structural estimator for productivity due to Olley and Pakes (1996) substantially decreases measured residual TFP. Combining labor quality and structural estimates of productivity, the one standard deviation difference in residual TFPs in manufacturing drops from 0.70 to 0.67 multiples. Neither the structural productivity measure nor detailed input quality measures explain the very large measured residual TFP dispersion, despite statistically precise coefficient estimatesproduction function estimation; total factor productivity; input quality; structural estimates of productivity

    Identifying Demand with Multidimensional Unobservables: A Random Functions Approach

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    We explore the identification of nonseparable models without relying on the property that the model can be inverted in the econometric unobservables. In particular, we allow for infinite dimensional unobservables. In the context of a demand system, this allows each product to have multiple unobservables. We identify the distribution of demand both unconditional and conditional on market observables, which allows us to identify several quantities of economic interest such as the (conditional and unconditional) distributions of elasticities and the distribution of price effects following a merger. Our approach is based on a significant generalization of the linear in random coefficients model that only restricts the random functions to be analytic in the endogenous variables, which is satisfied by several standard demand models used in practice. We assume an (unknown) countable support for the the distribution of the infinite dimensional unobservables.

    Efficiency enhancement of octave-bandwidth TWT's by the use of multistage depressed collectors

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    A small size two and four stage depressed collector was evaluated in conjunction with a 4.8 to 9.6 GHz traveling wave tube of 325 to 675 W power output. In spite of large fixed traveling wave tube losses, very significant efficiency enhancement was demonstrated with both the two and four stage multistage depressed collectors (MDC). The improvement in the overall efficiency ranged from a factor of 2.5 to 3.0 for the two stage MDC and a factor of 3.0 to 3.5 for the four stage MDC. An overall efficiency of 37.0 to 44.3 percent across the frequency band was demonstrated with the use of harmonic injection

    Efficiency enhancement of octave-bandwidth traveling wave tubes by use of multistage depressed collectors

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    Small, three- and five-stage depressed collectors were evaluated in conjunction with a 4.8- to 9.6-GHz TWT of 325- to 675-W power output and a beam of 0.5 microperv. The multistage depressed collector (MDC) performed well even though its design had been optimized for a TWT of identical design but considerably less output power. Despite large, fixed losses significant efficiency enhancement was demonstrated with both the three- and five-stage depressed collectors. At saturated rf power output, the improvement in the overall efficiency ranged from a factor of 2.5 to 3.0 for the three-stage collector and a factor of 3.0 to 3.5 for the five-stage collector. At saturation three-stage collector efficiencies of 77 to 80 percent and five-stage collector efficiencies of 81 to 84 percent were obtained across the frequency band. An overall efficiency of 37.0 to 44.3 percent across the frequency band of 4.8 to 9.6 GHz was demonstrated with the use of harmonic injection. For operation below saturation, even larger relative improvements in the overall TWT efficiency were demonstrated. Collector performance was relatively insensitive to the degree of regulation of the collector power supply
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