Showing papers by "University of Maryland, College Park published in 1998"

Evidence for oscillation of atmospheric neutrinos

Abstract: We present an analysis of atmospheric neutrino data from a 33.0 kton yr (535-day) exposure of the Super-Kamiokande detector. The data exhibit a zenith angle dependent deficit of muon neutrinos which is inconsistent with expectations based on calculations of the atmospheric neutrino flux. Experimental biases and uncertainties in the prediction of neutrino fluxes and cross sections are unable to explain our observation. The data are consistent, however, with two-flavor ${\ensuremath{ u}}_{\ensuremath{\mu}}\ensuremath{\leftrightarrow}{\ensuremath{ u}}_{\ensuremath{\tau}}$ oscillations with ${sin}^{2}2\ensuremath{\theta}g0.82$ and $5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}l\ensuremath{\Delta}{m}^{2}l6\ifmmode\times\else\texttimes\fi{}1{0}^{\ensuremath{-}3}\mathrm{eV}{}^{2}$ at 90% confidence level.

An optimal algorithm for approximate nearest neighbor searching fixed dimensions

Abstract: Consider a set of S of n data points in real d-dimensional space, Rd, where distances are measured using any Minkowski metric. In nearest neighbor searching, we preprocess S into a data structure, so that given any query point q∈ Rd, is the closest point of S to q can be reported quickly. Given any positive real ϵ, data point p is a (1 +ϵ)-approximate nearest neighbor of q if its distance from q is within a factor of (1 + ϵ) of the distance to the true nearest neighbor. We show that it is possible to preprocess a set of n points in Rd in O(dn log n) time and O(dn) space, so that given a query point q ∈ Rd, and ϵ > 0, a (1 + ϵ)-approximate nearest neighbor of q can be computed in O(cd, ϵ log n) time, where cd,ϵ≤d ⌈1 + 6d/ϵ⌉d is a factor depending only on dimension and ϵ. In general, we show that given an integer k ≥ 1, (1 + ϵ)-approximations to the k nearest neighbors of q can be computed in additional O(kd log n) time.

Law, finance, and firm growth

Abstract: We investigate how differences in legal and financial systems affect firms' use of external financing to fund growth. We show that in countries whose legal systems score high on an efficiency index, a greater proportion of firms use long-term external financing. An active, though not necessarily large, stock market and a large banking sector are also associated with externally financed firm growth. The increased reliance on external financing occurs in part because established firms in countries with well-functioning institutions have lower profit rates. Government subsidies to industry do not increase the proportion of firms relying on external financing. THE CORPORATE FINANCE LITERATURE suggests that market imperfections, caused by conflicts of interest and informational asymmetries between corporate insiders and investors, constrain firms in their ability to fund investment projects. The magnitude of these imperfections depends in part on the effectiveness of the legal and financial systems. Because these systems differ across countries, the literature implies that there should exist systematic cross-country differences in firms' ability to obtain external capital to finance investment. In this paper, we examine whether the underdevelopment of legal and financial systems does prevent firms in some countries from investing in potentially profitable growth opportunities. In particular, we focus on the use of long-term debt or external equity to fund growth (see our earlier work, Demirgut-Kunt and Maksimovic (1996a), which compares firms' financial structures in developed and developing countries and finds the greatest difference to be in the provision of long-term credit). We estimate a financial planning model to obtain the maximum growth rate that each firm in our thirty-country sample could attain without access to long-term financing. We then compare these predicted growth rates to growth rates realized by firms in countries with differing degrees of development in their legal and financial systems. Our approach enables us to identify specific characteristics of the legal and financial systems that are associated with long-term financing of firm growth. Thus, we provide a micro-level test of the hypoth

Using the correct statistical test for the equality of regression coefficients

Abstract: Criminologists are often interested in examining interactive effects within a regression context. For example, “holding other relevant factors constant, is the effect of delinquent peers on one's own delinquent conduct the same for males and females?” or “is the effect of a given treatment program comparable between first-time and repeat offenders?” A frequent strategy in examining such interactive effects is to test for the difference between two regression coefficients across independent samples. That is, does b1= b2? Traditionally, criminologists have employed a t or z test for the difference between slopes in making these coefficient comparisons. While there is considerable consensus as to the appropriateness of this strategy, there has been some confusion in the criminological literature as to the correct estimator of the standard error of the difference, the standard deviation of the sampling distribution of coefficient differences, in the t or z formula. Criminologists have employed two different estimators of this standard deviation in their empirical work. In this note, we point out that one of these estimators is correct while the other is incorrect. The incorrect estimator biases one's hypothesis test in favor of rejecting the null hypothesis that b1= b2. Unfortunately, the use of this incorrect estimator of the standard error of the difference has been fairly widespread in criminology. We provide the formula for the correct statistical test and illustrate with two examples from the literature how the biased estimator can lead to incorrect conclusions.

A Neoproterozoic Snowball Earth

Abstract: Negative carbon isotope anomalies in carbonate rocks bracketing Neoproterozoic glacial deposits in Namibia, combined with estimates of thermal subsidence history, suggest that biological productivity in the surface ocean collapsed for millions of years. This collapse can be explained by a global glaciation (that is, a snowball Earth), which ended abruptly when subaerial volcanic outgassing raised atmospheric carbon dioxide to about 350 times the modern level. The rapid termination would have resulted in a warming of the snowball Earth to extreme greenhouse conditions. The transfer of atmospheric carbon dioxide to the ocean would result in the rapid precipitation of calcium carbonate in warm surface waters, producing the cap carbonate rocks observed globally.

The Variance Gamma Process and Option Pricing

Abstract: A three parameter stochastic process, termed the variance gamma process, that generalizes Brownian motion is developed as a model for the dynamics of log stock prices. The process is obtained by evaluating Brownian motion with drift at a random time given by a gamma process. The two additional parameters are the drift of the Brownian motion and the volatility of the time change. These additional parameters provide control over the skewness and kurtosis of the return distribution. Closed forms are obtained for the return density and the prices of European options. The statistical and risk neutral densities are estimated for data on the S&P500 Index and the prices of options on this Index. It is observed that the statistical density is symmetric with some kurtosis, while the risk neutral density is negatively skewed with a larger kurtosis. The additional parameters also correct for pricing biases of the Black Scholes model that is a parametric special case of the option pricing model developed here.

A Generalized Spatial Two-Stage Least Squares Procedure for Estimating a Spatial Autoregressive Model with Autoregressive Disturbances

Abstract: Cross-sectional spatial models frequently contain a spatial lag of the dependent variable as a regressor or a disturbance term that is spatially autoregressive. In this article we describe a computationally simple procedure for estimating cross-sectional models that contain both of these characteristics. We also give formal large-sample results.

A Dynamic Model of the Duration of the Customer's Relationship with a Continuous Service Provider: The Role of Satisfaction

Abstract: Many service organizations have embraced relationship marketing with its focus on maximizing customer lifetime value. Recently, there has been considerable controversy about whether there is a link between customer satisfaction and retention. This research question is important to researchers who are attempting to understand how customers' assessments of services influence their subsequent behavior. However, it is equally vital to managers who require a better understanding of the relationship between satisfaction and the duration of the provider-customer relationship to identify specific actions that can increase retention and profitability in the long run. Since there is very little empirical evidence regarding this research question, this study develops and estimates a dynamic model of the duration of provider-customer relationship that focuses on the role of customer satisfaction. This article models the duration of the customer's relationship with an organization that delivers a continuously provided service, such as utilities, financial services, and telecommunications. In the model, the duration of the provider-customer relationship is postulated to depend on the customer's subjective expected value of the relationship, which he/she updates according to an anchoring and adjustment process. It is hypothesized that cumulative satisfaction serves as an anchor that is updated with new information obtained during service experiences. The model is estimated as a left-truncated, proportional hazards regression with cross-sectional and time series data describing cellular customers perceptions and behavior over a 22-month period. The results indicate that customer satisfaction ratings elicited prior to any decision to cancel or stay loyal to the provider are positively related to the duration of the relationship. The strength of the relationship between duration times and satisfaction levels depends on the length of customers' prior experience with the organization. Customers who have many months' experience with the organization weigh prior cumulative satisfaction more heavily and new information relatively less heavily. The duration of the service provider-customer relationship also depends on whether customers experienced service transactions or failures. The effects of perceived losses arising from transactions or service failures on duration times are directly weighed by prior satisfaction, creating contrast and assimilation effects. How can service organizations develop longer relationships with customers? Since customers weigh prior cumulative satisfaction heavily, organizations should focus on customers in the early stages of the relationship-if customers' experiences are not satisfactory, the relationship is likely to be very short. There is considerable heterogeneity across customers because some customers have a higher utility for the service than others. However, certain types of service encounters are potential relationship "landmines" because customers are highly sensitive to the costs/losses arising from interactions with service organizations and insensitive to the benefits/gains. Thus, incidence and quality of service encounters can be early indicators of whether an organization's relationship with a customer is flourishing or in jeopardy. Unfortunately, organizations with good prior service levels will suffer more when customers perceive that they have suffered a loss arising from a service encounter-due to the existence of contrast effects. However, experienced customers are less sensitive to such losses because they tend to weigh prior satisfaction levels heavily. By modeling the duration of the provider-customer relationship, it is possible to predict the revenue impact of service improvements in the same manner as other resource allocation decisions. The calculations in this article show that changes in customer satisfaction can have important financial implications for the organization because lifetime revenues from an individual customer depend on the duration of his/her relationship, as well as the dollar amount of his/her purchases across billing cycles. Satisfaction levels explain a substantial portion of explained variance in the durations of service provider-customer relationships across customers, comparable to the effect of price. Consequently, it is a popular misconception that organizations that focus on customer satisfaction are failing to manage customer retention. Rather, this article suggests that service organizations should be proactive and learn from customers before they defect by understanding their current satisfaction levels. Managers and researchers may have underestimated the importance of the link between customer satisfaction and retention because the relationship between satisfaction and duration times is very complex and difficult to detect without advanced statistical techniques.

Linking Service Climate and Customer Perceptions of Service Quality: Test of a Causal Model

Abstract: A set of foundation issues that support employee work and service quality is conceptualized as a necessary but not sufficient cause of a climate for service, which in turn is proposed to be reflected in customer experiences. Climate for service rests on the foundation issues, but in addition it requires policies and practices that focus attention directly on service quality. Data were collected at multiple points in time from employees and customers of 134 branches of a bank and analyzed via structural equation modeling. Results indicated that the model in which the foundation issues yielded a climate for service, and climate for service in turn led to customer perceptions of service quality, fit the data well. However, subsequent cross-lagged analyses revealed the presence of a reciprocal effect for climate and customer perceptions. Implications of these results for theory and research are offered.

Evolutionarily singular strategies and the adaptive growth and branching of the evolutionary tree

Abstract: Summary We present a general framework for modelling adaptive trait dynamics in which we integrate various concepts and techniques from modern ESS-theory The concept of evolutionarily singular strategies is introduced as a generalization of the ESS-concept We give a full classification of the singular strategies in terms of ESSstability, convergence stability, the ability of the singular strategy to invade other populations if initially rare itself, and the possibility of protected dimorphisms occurring within the singular strategy’s neighbourhood Of particular interest is a type of singular strategy that is an evolutionary attractor from a great distance, but once in its neighbourhood a population becomes dimorphic and undergoes disruptive selection leading to evolutionary branching Modelling the adaptive growth and branching of the evolutionary tree can thus be considered as a major application of the framework A haploid version of Levene’s ‘soft selection’ model is developed as a specific example to demonstrate evolutionary dynamics and branching in monomorphic and polymorphic populations

Social Relationships and Motivation in Middle School: The Role of Parents, Teachers, and Peers

Abstract: Adolescents' supportive relationships with parents, teachers, and peers were examined in relation to motivation at school (school- and class-related interest, academic goal orientations, and social goal pursuit). On the basis of 167 sixth-grade students, relations of perceived support from parents, teachers, and peers to student motivation differed depending on the source of support and motivational outcome: Peer support was a positive predictor of prosocial goal pursuit, teacher support was a positive predictor of both types of interest and of social responsibility goal pursuit, and parent support was a positive predictor of school-related interest and goal orientations. Perceived support from parents and peers also was related to interest in school indirectly by way of negative relations with emotional distress. Pursuit of social responsibility goals and school- and class-related interest in 6th grade partly explained positive relations between social support in 6th grade and classroom grades 1 year later. Continued research on the social origins of classroom motivation in early adolescence is needed. For most students, early adolescence is a time of change and transition. With respect to interpersonal relationships and social adjustment, these changes reflect a growing psychological and emotional independence from adults and a corresponding dependence on peer relationships to establish and maintain positive perceptions of the self (Steinberg, 1990; Youniss & Smollar, 1985). Often confounding these general developmental challenges is a transition to a new school environment, which tends to be marked by adolescents' perceptions that teachers no longer care about them, and decreased opportunities to establish meaningful relationships with peers (Eccles & Midgley, 1989). Therefore, young adolescents often must negotiate and establish relationships with adults and peers under less than optimal conditions. A particular concern is that young adolescents who do not enjoy positive, supportive relationships with adults and peers are often at risk for academic problems (e.g., Goodenow, 1993; Midgley, Feldlaufer, & Eccles, 1989; Phelan, Davidson, & Cao, 1991). In the present study, I examined specific ways in which supportive relationships with parents, teachers, and peers are related to young adolescents' motivation at school and to academic performance.

The Moderate Resolution Imaging Spectroradiometer (MODIS): land remote sensing for global change research

Abstract: The first Moderate Resolution Imaging Spectroradiometer (MODIS) instrument is planned for launch by NASA in 1998. This instrument will provide a new and improved capability for terrestrial satellite remote sensing aimed at meeting the needs of global change research. The MODIS standard products will provide new and improved tools for moderate resolution land surface monitoring. These higher order data products have been designed to remove the burden of certain common types of data processing from the user community and meet the more general needs of global-to-regional monitoring, modeling, and assessment. The near-daily coverage of moderate resolution data from MODIS, coupled with the planned increase in high-resolution sampling from Landsat 7, will provide a powerful combination of observations. The full potential of MODIS will be realized once a stable and well-calibrated time-series of multispectral data has been established. In this paper the proposed MODIS standard products for land applications are described along with the current plans for data quality assessment and product validation.

Corporate governance and board effectiveness

Abstract: This paper surveys the empirical and theoretical literature on the mechanisms of corporate governance. We focus on the internal mechanisms of corporate governance (e.g., corporate board of directors) and their role in ameliorating various classes of agency problems arising from conflicts of interests between managers and equityholders, equityholders and creditors, and capital contributors and other stakeholders to the corporate firm. We also examine the substitution effect between internal mechanisms of corporate governance and external mechanisms, particularly markets for corporate control. Directions for future research are provided.

Aggregate Productivity Growth: Lessons from Microeconomic Evidence

Abstract: In this paper, we exploit establishment-level data to examine the relationship between microeconomic productivity dynamics and aggregate productivity growth. After synthesizing the evidence from recent studies, we conduct our own analysis using establishment-level data for U.S. manufacturing establishments as well for selected service industries. The use of longitudinal micro data on service sector establishments is one of the novel features of our analysis. Our main findings are summarized as follows: (i) the contribution of reallocation of outputs and inputs from less productive to more productive establishments plays a significant role in accounting for aggregate productivity growth; (ii) for the selected service industries considered, the contribution of net entry (more productive entering establishments displacing less productive exiting establishments) is dominant; (iii) the contribution of net entry to aggregate productivity growth is disproportionate and is increasing in the horizon over which the changes are measured since longer horizon yields greater differentials from selection and learning effects; (iv) the contribution of reallocation to aggregate productivity growth varies over time (e.g. is cyclically sensitive) and industries and is somewhat sensitive to subtle differences in measurement and decomposition methodologies.

Volume of the set of separable states

Abstract: The question of how many entangled or, respectively, separable states there are in the set of all quantum states is considered. We propose a natural measure in the space of density matrices $\ensuremath{\varrho}$ describing $N$-dimensional quantum systems. We prove that, under this measure, the set of separable states possesses a nonzero volume. Analytical lower and upper bounds of this volume are also derived for $N=2\ifmmode\times\else\texttimes\fi{}2$ and $N=2\ifmmode\times\else\texttimes\fi{}3$ cases. Finally, numerical Monte Carlo calculations allow us to estimate the volume of separable states, providing numerical evidence that it decreases exponentially with the dimension of the composite system. We have also analyzed a conditional measure of separability under the condition of fixed purity. Our results display a clear dualism between purity and separability: entanglement is typical of pure states, while separability is connected with quantum mixtures. In particular, states of sufficiently low purity are necessarily separable.

Gaugino mass without singlets

Abstract: In models with dynamical supersymmetry breaking in the hidden sector, the gaugino masses in the observable sector have been believed to be extremely suppressed (below 1 keV), unless there is a gauge singlet in the hidden sector with specific couplings to the observable sector gauge multiplets. We point out that there is a pure supergravity contribution to gaugino masses at the quantum level arising from the superconformal anomaly. Our results are valid to all orders in perturbation theory and are related to the ‘exact’ beta functions for soft terms. There is also an anomaly contribution to the A terms proportional to the beta function of the corresponding Yukawa coupling. The gaugino masses are proportional to the corresponding gauge beta functions, and so do not satisfy the usual GUT relations.

Direct evidence for a half-metallic ferromagnet

Abstract: Half-metallic materials are characterized by the coexistence of metallic behaviour for one electron spin and insulating behaviour for the other. Thus, the electronic density of states is completely spin polarized at the Fermi level, and the conductivity is dominated by these metallic single-spin charge carriers. This exotic physical property could have a significant effect on technological applications related to magnetism and spin electronics. Some ferromagnetic systems, such as Heusler compounds1 and chromium dioxide2, have been predicted theoretically to be half-metallic. However, a half-metallic system has not been demonstrated directly and the predictions are still in doubt3,4. Here we report spin-resolved photoemission measurements of a ferromagnetic manganese perovskite, La0.7Sr0.3MnO3, which directly manifest the half-metallic nature well below the Curie temperature. For the majority spin, the photoemission spectrum clearly shows a metallic Fermi cut-off, whereas for the minority spin, it shows an insulating gap with disappearance of spectral weight at ∼0.6 eV binding energy.

Sulfur Dioxide Control by Electric Utilities: What Are the Gains from Trade?

Abstract: Title IV of the 1990 U.S. Clean Air Act Amendments offers firms facing high marginal costs for pollution abatement the chance to purchase the right to emit sulfur dioxide from firms with lower costs. In the long run such allowance trading may achieve substantial cost savings over an "enlightened" command and control program with a uniform emission-rate standard. But in the short run what has lowered costs is technical change and the fall in low-sulfur coal prices. Title IV of the 1990 U.S. Clean Air Act Amendments established a market for transferable sulfur dioxide emission allowances among electric utilities. The market offers firms facing high marginal costs for pollution abatement the opportunity to purchase the right to emit sulfur dioxide from firms with lower costs. It is expected to yield more cost savings than a command and control approach to environmental regulation. To evaluate the performance of the market for sulfur dioxide allowances, Carlson, Burtraw, Cropper, and Palmer use econometrically estimated marginal abatement cost functions for power plants affected by Title IV. They investigate whether the much-heralded fall in the cost of abating sulfur dioxide can be attributed to allowance trading. They find that for plants that use low-sulfur coal to reduce sulfur dioxide emissions, technical change and the fall in low-sulfur coal prices have lowered marginal abatement cost curves by more than half since 1985. And that is the main source of cost reductions rather than trading allowances per se. In the long run, allowance trading may achieve cost savings of $700 million to $800 million a year more than could be expected from an "enlightened" command and control program with a uniform emission-rate standard. But comparing potential cost savings in 1995 and 1996 with actual emissions costs suggests that most trading gains were unrealized in the first two years of the program. This paper-a product of Environment and Infrastructure, Development Research Group-is part of a larger effort in the group to examine the successes and failures of environmental regulation as a guide to formulating environmental policy. Maureen Cropper may be contacted at mcropper@worldbank.org

Mutual Fund Herding and the Impact on Stock Prices

Abstract: We analyze the trading activity of the mutual fund industry between 1975 and 1994 to determine whether funds "herd" when they trade stocks and to investigate the impact of herding on stock prices. Although we find little herding by mutual funds in the average stock, we find much higher levels in trades of small stocks and in trading by growth-oriented funds. Stocks that herds buy outperform stocks that they sell by four percent during the following six months; this return difference is much more pronounced among small stocks. Our results are consistent with mutual fund herding speeding the price-adjustment process.

Profit, directional distance functions, and Nerlovian efficiency

Abstract: The directional technology distance function is introduced, given an interpretation as a min-max, and compared with other functional representations of the technology including the Shephard input and output distance functions and the McFadden gauge function. A dual correspondence is developed between the directional technology distance function and the profit function, and it is shown that all previous dual correspondences are special cases of this correspondence. We then show how Nerlovian (profit-based) efficiency measures can be computed using the directional technology distance function.

Genes and enzymes of carotenoid biosynthesis in plants

Abstract: Carotenoids are integral and essential components of the photosynthetic membranes in all plants. Within the past few years, genes encoding nearly all of the enzymes required for the biosynthesis of these indispensable pigments have been identified. This review focuses on recent findings as to the structure and function of these genes and the enzymes they encode. Three topics of current interest are also discussed: the source of isopentenyl pyrophosphate for carotenoid biosynthesis, the progress and possibilities of metabolic engineering of plants to alter carotenoid content and composition, and the compartmentation and association of the carotenogenic enzymes. A speculative schematic model of carotenogenic enzyme complexes is presented to help frame and provoke insightful questions leading to future experimentation.

A dynamic regulatory model of phytoplanktonic acclimation to light, nutrients, and temperature

Abstract: A new regulatory model can describe acclimation of phytoplankton growth rate (p), chlorophyll a : carbon ratio and nitrogen: carbon ratio to irradiance, temperature and nutrient availability. The model uses three indices of phytoplankton biomass-phytoplankton carbon (C), phytoplankton nitrogen (N), and chlorophyll a (Chl). The model links the light-saturated rate of photosynthesis to N: C, requires that Chl a synthesis be coupled to nitrogen assimilation, and includes several regulatory features. These include feedback inhibition of the nitrogen assimilation rate by increases in the N: C ratio, as well as regulation of Chl a synthesis by the balance between light absorption and photosynthetic carbon fixation. The model treats respiration as the sum of the maintenance metabolic requirement and the cost of biosynthesis. In addition, the model can account for accumulation and mobilization of energy reserves (i.e. variability of N: C) and photoacclimation (i.e. variability of Chl : N and Chl : C) in response to variations in irradiance and nutrient availability. The assumptions of the model are shown to be in agreement with experimental observations and the model output compares favorably with data for cultures in balanced and unbalanced growth. The light-, nutrient-, and temperature-dependencies of phytoplankton growth rate have often been modeled separately. Phytoplankton growth rate has been treated as a function of the dissolved nutrient concentration using the Monod equation or as a function of the cellular content (or quota) of limiting nutrient using the Droop equation (McCarthy 1980; Droop 1983; Morel 1987). Under conditions of balanced growth, the Droop and Monod equations are consistent with each other and with Michaelis-Menten nutrient uptake kinetics (Morel 1987). The temperature-dependence of growth rate has been treated as an exponential dependence or an Arrhenius equation, although other functions have also been used (Eppley 1972; Li 1980; Ahlgren 1987). The lightdependencies of growth and photosynthesis rates have been treated by a number of equations, including a modification of the Monod equation, a hyperbolic tangent, and a Poisson function