Showing papers by "University of Maryland, College Park published in 1994"

Highly dynamic Destination-Sequenced Distance-Vector routing (DSDV) for mobile computers

Abstract: An ad-hoc network is the cooperative engagement of a collection of Mobile Hosts without the required intervention of any centralized Access Point. In this paper we present an innovative design for the operation of such ad-hoc networks. The basic idea of the design is to operate each Mobile Host as a specialized router, which periodically advertises its view of the interconnection topology with other Mobile Hosts within the network. This amounts to a new sort of routing protocol. We have investigated modifications to the basic Bellman-Ford routing mechanisms, as specified by RIP [5], to make it suitable for a dynamic and self-starting network mechanism as is required by users wishing to utilize ad hoc networks. Our modifications address some of the previous objections to the use of Bellman-Ford, related to the poor looping properties of such algorithms in the face of broken links and the resulting time dependent nature of the interconnection topology describing the links between the Mobile Hosts. Finally, we describe the ways in which the basic network-layer routing can be modified to provide MAC-layer support for ad-hoc networks.

Fast subsequence matching in time-series databases

Abstract: We present an efficient indexing method to locate 1-dimensional subsequences within a collection of sequences, such that the subsequences match a given (query) pattern within a specified tolerance. The idea is to map each data sequences into a small set of multidimensional rectangles in feature space. Then, these rectangles can be readily indexed using traditional spatial access methods, like the R*-tree [9]. In more detail, we use a sliding window over the data sequence and extract its features; the result is a trail in feature space. We propose an efficient and effective algorithm to divide such trails into sub-trails, which are subsequently represented by their Minimum Bounding Rectangles (MBRs). We also examine queries of varying lengths, and we show how to handle each case efficiently. We implemented our method and carried out experiments on synthetic and real data (stock price movements). We compared the method to sequential scanning, which is the only obvious competitor. The results were excellent: our method accelerated the search time from 3 times up to 100 times.

Momentum Investment Strategies, Portfolio Performance, and Herding: A Study of Mutual Fund Behavior

Abstract: We examine the investment strategies of 155 mutual funds over the 1975-84 period to determine the extent to which the funds purchased stocks based on their past returns, and to determine the relation of this behavior to their observed portfolio performance We find that about 77% of these mutual funds were "momentum investors", buying stocks that were past winners; however, they did not systematically sell past losers On average, these "trend-followers" realized significantly better performance than the remaining funds We also find that the mutual funds exhibited herding behavior, and that the tendency of a fund to herd in its trades was strongly correlated with its tendency to buy past winners as well as with its portfolio performance Consistent with the evidence on trend-following, herding into past winners was stronger than herding into past losers

The evolutionary dynamics of repetitive DNA in eukaryotes

Abstract: Repetitive DNA sequences form a large portion of the genomes of eukaryotes. The 'selfish DNA' hypothesis proposes that they are maintained by their ability to replicate within the genome. The behaviour of repetitive sequences can result in mutations that cause genetic diseases, and confer significant fitness losses on the organism. Features of the organization of repetitive sequences in eukaryotic genomes, and their distribution in natural populations, reflect the evolutionary forces acting on selfish DNA.

Levels Issues in Theory Development, Data Collection, and Analysis

Abstract: Despite past entreaties to organizational theorists and researchers to address levels issues more carefully, levels issues continue to arouse confusion and controversy within the organizational literature. We highlight three alternative assumptions that underlie the specification of levels of theory throughout organizational behavior: (a) homogeneity within higher level units, (b) independence from higher level units, and (c) heterogeneity within higher level units. These assumptions influence the nature of theoretical constructs and propositions and should, ideally, also influence data collection, analysis, and interpretation. Greater attention to levels issues will strengthen organizational theory development and research.

Efficient and effective querying by image content

Abstract: In the QBIC (Query By Image Content) project we are studying methods to query large on-line image databases using the images' content as the basis of the queries. Examples of the content we use include color, texture, shape, position, and dominant edges of image objects and regions. Potential applications include medical (“Give me other images that contain a tumor with a texture like this one”), photo-journalism (“Give me images that have blue at the top and red at the bottom”), and many others in art, fashion, cataloging, retailing, and industry. We describe a set of novel features and similarity measures allowing query by image content, together with the QBIC system we implemented. We demonstrate the effectiveness of our system with normalized precision and recall experiments on test databases containing over 1000 images and 1000 objects populated from commercially available photo clip art images, and of images of airplane silhouettes. We also present new methods for efficient processing of QBIC queries that consist of filtering and indexing steps. We specifically address two problems: (a) non Euclidean distance measures; and (b) the high dimensionality of feature vectors. For the first problem, we introduce a new theorem that makes efficient filtering possible by bounding the non-Euclidean, full cross-term quadratic distance expression with a simple Euclidean distance. For the second, we illustrate how orthogonal transforms, such as Karhunen Loeve, can help reduce the dimensionality of the search space. Our methods are general and allow some “false hits” but no false dismissals. The resulting QBIC system offers effective retrieval using image content, and for large image databases significant speedup over straightforward indexing alternatives. The system is implemented in X/Motif and C running on an RS/6000.

Top Management Team Demography and Process: The Role of Social Integration and Communication

Abstract: We thank Ed Locke, Martin Gannon, Marshall W. Meyer, Stefan Wally, and three anonymous reviewers for their helpful comments on earlier drafts of this paper, and the Dingman Center for Entrepreneurship at the University of Maryland for the-financial grant that made this research possible. This research used data from 53 high-technology firms to test three alternative models of the effects of the top management team's demography and process on organizational performance: (1) a demography model, in which team demography accounts entirely for performance outcomes, and process has no impact; (2) a process model, in which process contributes incrementally and directly to performance outcomes, over and above the team's demography; and (3) an intervening model, in which the effects of the top management team on performance outcomes are due entirely to the effects of its demography on process. The study found the top management team's demography indirectly related to performance through process and process directly related to performance, although direct effects of team demography on performance were also found. These results suggest a fourth, more complex model of top management team behavior.'

Solution structure of a calmodulin-target peptide complex by multidimensional NMR.

Abstract: The three-dimensional solution structure of the complex between calcium-bound calmodulin (Ca(2+)-CaM) and a 26-residue synthetic peptide comprising the CaM binding domain (residues 577 to 602) of skeletal muscle myosin light chain kinase, has been determined using multidimensional heteronuclear filtered and separated nuclear magnetic resonance spectroscopy. The two domains of CaM (residues 6 to 73 and 83 to 146) remain essentially unchanged upon complexation. The long central helix (residues 65 to 93), however, which connects the two domains in the crystal structure of Ca(2+)-CaM, is disrupted into two helices connected by a long flexible loop (residues 74 to 82), thereby enabling the two domains to clamp residues 3 to 21 of the bound peptide, which adopt a helical conformation. The overall structure of the complex is globular, approximating an ellipsoid of dimensions 47 by 32 by 30 angstroms. The helical peptide is located in a hydrophobic channel that passes through the center of the ellipsoid at an angle of approximately 45 degrees with its long axis. The complex is mainly stabilized by hydrophobic interactions which, from the CaM side, involve an unusually large number of methionines. Key residues of the peptide are Trp4 and Phe17, which serve to anchor the amino- and carboxyl-terminal halves of the peptide to the carboxyl- and amino-terminal domains of CaM, respectively. Sequence comparisons indicate that a number of peptides that bind CaM with high affinity share this common feature containing either aromatic residues or long-chain hydrophobic ones separated by a stretch of 12 residues, suggesting that they interact with CaM in a similar manner.

Expectancy-Value Theory of Achievement Motivation: A Developmental Perspective

Abstract: I consider Eccles et al.'s (1983) expectancy-value model of achievement performance and choice from a developmental perspective, by examining how recent research on the development of young children's competence beliefs, expectancies for success, subjective task values, and achievement goals can be incorporated into the model. The kinds of change in children's achievement beliefs considered include change in the factor structure of children's competence beliefs and values; change across age in the mean level of those constructs; and change in children's conceptions of ability beliefs and subjective values. I also discuss how achievement goals are conceptualized in this model, and how goals are conceived by other current motivation researchers. Changes in the nature of relations among competence beliefs, subjective task values, achievement goals, and achievement behaviors also are considered.

Visual information seeking: tight coupling of dynamic query filters with starfield displays

Abstract: This paper offers new principles for visual information seeking (VIS). A key concept is to support browsing, which is distinguished from familiar query composition and information retrieval because of its emphasis on rapid filtering to reduce result sets, progressive refinement of search parameters, continuous reformulation of goals, and visual scanning to identify results. VIS principles developed include: dynamic query filters (query parameters are rapidly adjusted with sliders, buttons, maps, etc.), starfield displays (two-dimensional scatterplots to structure result sets and zooming to reduce clutter), and tight coupling (interrelating query components to preserve display invariants and support progressive refinement combined with an emphasis on using search output to foster search input). A FilmFinder prototype using a movie database demonstrates these principles in a VIS environment.

NDVI-derived land cover classifications at a global scale

Abstract: Phenological differences among vegetation types, reflected in temporal variations in the Normalized Difference Vegetation Index (NDVI) derived from satellite data, have been used to classify land cover at continental scales. Extending this technique to global scales raises several issues: identifying land cover types that are spectrally distinct and applicable at the global scale; accounting for phasing of seasons in different parts of the world; validating results in the absence of reliable information on global land cover; and acquiring high quality global data sets of satellite sensor data for input to land cover classifications. For this study, a coarse spatial resolution (one by one degree) data set of monthly NDVI values for 1987 was used to explore these methodological issues. A result of a supervised, maximum likelihood classification of eleven cover types is presented to illustrate the feasibility of using satellite sensor data to increase the accuracy of global land cover information, a...

On black hole entropy

Abstract: Two techniques for computing black hole entropy in generally covariant gravity theories including arbitrary higher derivative interactions are studied. The techniques are Wald's Noether charge approach introduced recently, and a field redefinition method developed in this paper. Wald's results are extended by establishing that his local geometric expression for the black hole entropy gives the same result when evaluated on an arbitrary cross section of a Killing horizon (rather than just the bifurcation surface). Further, we show that his expression for the entropy is not affected by ambiguities which arise in the Noether construction. Using the Noether charge expression, the entropy is evaluated explicitly for black holes in a wide class of generally covariant theories. For a Lagrangian of the functional form L\ifmmode \tilde{}\else \~{}\fi{}=L\ifmmode \tilde{}\else \~{}\fi{}(${\mathrm{\ensuremath{\psi}}}_{\mathit{m}}$, ${\mathrm{\ensuremath{ abla}}}_{\mathit{a}}$${\mathrm{\ensuremath{\psi}}}_{\mathit{m}}$,${\mathit{g}}_{\mathit{a}\mathit{b}}$,${\mathit{R}}_{\mathit{a}\mathit{b}\mathit{c}\mathit{d}}$, ${\mathrm{\ensuremath{ abla}}}_{\mathit{e}}$${\mathit{R}}_{\mathit{a}\mathit{b}\mathit{c}\mathit{d}}$), it is found that the entropy is given by S=-2\ensuremath{\pi}\ensuremath{\oint}(${\mathit{Y}}^{\mathit{a}\mathit{b}\mathit{c}\mathit{d}}$-${\mathrm{\ensuremath{ abla}}}_{\mathit{e}}$${\mathit{Z}}^{\mathit{e}:\mathit{a}\mathit{b}\mathit{c}\mathit{d}}$) \ensuremath{\epsilon}${\mathrm{^}}_{\mathit{a}\mathit{b}}$\ensuremath{\epsilon}${\mathrm{^}}_{\mathit{c}\mathit{d}}$\ensuremath{\epsilon}\ifmmode\bar\else\textasciimacron\fi{}, where the integral is over an arbitrary cross section of the Killing horizon, \ensuremath{\epsilon}${\mathrm{^}}_{\mathit{a}\mathit{b}}$ is the binormal to the cross section, ${\mathit{Y}}^{\mathit{a}\mathit{b}\mathit{c}\mathit{d}}$=\ensuremath{\partial}L\ifmmode \tilde{}\else \~{}\fi{}/\ensuremath{\partial}${\mathit{R}}_{\mathit{a}\mathit{b}\mathit{c}\mathit{d}}$, and ${\mathit{Z}}^{\mathit{e}:\mathit{a}\mathit{b}\mathit{c}\mathit{d}}$=\ensuremath{\partial}L\ifmmode \tilde{}\else \~{}\fi{}/\ensuremath{\partial}${\mathrm{\ensuremath{ abla}}}_{\mathit{e}}$${\mathit{R}}_{\mathit{a}\mathit{b}\mathit{c}\mathit{d}}$.Further, it is shown that the Killing horizon and surface gravity of a stationary black hole metric are invariant under field redefinitions of the metric of the form g${\mathrm{\ifmmode\bar\else\textasciimacron\fi{}}}_{\mathit{a}\mathit{b}}$\ensuremath{\equiv}${\mathit{g}}_{\mathit{a}\mathit{b}}$+${\mathrm{\ensuremath{\Delta}}}_{\mathit{a}\mathit{b}}$, where ${\mathrm{\ensuremath{\Delta}}}_{\mathit{a}\mathit{b}}$ is a stationary tensor field that vanishes at infinity and is regular on the horizon (including the bifurcation surface). Using this result, a technique is developed for evaluating the black hole entropy in a given theory in terms of that of another theory related by field redefinitions. Remarkably, it is established that certain perturbative, first order, results obtained with this method are in fact exact. A particular result established in this fashion is that a scalar matter term of the form ${\mathrm{\ensuremath{ abla}}}^{2\mathit{p}}$\ensuremath{\varphi}${\mathrm{\ensuremath{ abla}}}^{2\mathit{q}}$\ensuremath{\varphi} in the Lagrangian makes no contribution to the black hole entropy. The possible significance of these results for the problem of finding the statistical origin of black hole entropy is discussed.

The Anderson-Mott transition

Abstract: The interacting disordered electron problem is reviewed with emphasis on the quantum phase transitions that occur in a model system and on the field-theoretic methods used to describe them. An elementary discussion of conservation laws and diffusive dynamics is followed by a detailed derivation of the extended nonlinear sigma model, which serves as an effective field theory for the problem. A general scaling theory of metal-insulator and related transitions is developed, and explicit renormalization-group calculations for the various universality classes are reviewed and compared with experimental results. A discussion of pertinent physical ideas and phenomenological approaches to the metal-insulator transition not contained in the sigma-model approach is given, and phase-transition aspects of related problems, like disordered superconductors and the quantum Hall effect, are discussed. The review concludes with a list of open problems.

HTN planning: complexity and expressivity

Abstract: Most practical work on AI planning systems during the last fifteen years has been based on hierarchical task network (HTN) decomposition, but until now, there has been very little analytical work on the properties of HTN planners. This paper describes how the complexity of HTN planning varies with various conditions on the task networks.

Expatriate managers and the psychological contract.

Abstract: The authors investigate employer practices toward expatriate managers and how those practices relate to retention-relevant outcomes (e.g., organizational commitment, intent to quit, and intent to return early to a domestic assignment). The psychological contract is investigated as a mediator of this relationship. A model in which the psychological contract mediates the relationship between organizational practices and retention-relevant outcomes is empirically constructed. The authors also refine the construct of the psychological contract and report managers' experiences as expatriates

Annual flux of dissolved organic carbon from the euphotic zone in the northwestern Sargasso Sea

Abstract: THE export of biogenic carbon from the upper ocean is responsible for maintaining the vertical gradient of dissolved inorganic carbon and thus indirectly for regulating the level of atmospheric CO2 (ref. 1). Large, rapidly sinking particles are thought to dominate this export2, and this sinking flux has been thought to balance new production3. Recent measurements of particle export4—6 and estimates of new production7—9 have questioned this picture, however. Here we report measurements of dissolved organic carbon (DOC) off Bermuda, which provide strong support for the idea10—15 that this component of oceanic carbon is also an important and dynamic part of the ocean carbon cycle. We find that DOC accumulates in the early spring owing to increased primary production, and is partially consumed in the summer and autumn. The DOC that escapes remineralization is exported from the surface ocean the following winter, and we estimate this export to be equal to or greater than the measured particle flux, allowing us to close the annual vertical carbon budget for this site to within a factor of two. Our observations should be applicable to other temperate, sub-polar and continental-shelf regions of the world ocean which exhibit convective mixing and vernal restratification.

Broadcast Disks: Data Management for Asymmetric Communication Environments.

Abstract: This paper proposes the use of repetitive broadcast as a way of augmenting the memory hierarchy of clients in an asymmetric communication environment. We describe a new technique called "Broadcast Disks" for structuring the broadcast in a way that provides improved performance for non-uniformly accessed data. The Broadcast Disk superimposes multiple disks spinning at different speeds on a single broadcast channel--in effect creating an arbitrarily fine-grained memory hierarchy. In addition to proposing and defining the mechanism, a main result of this work is that exploiting the potential of the broadcast structure requires a re-evaluation of basic cache management policies. We examine several "pure" cache management policies and develop and measure implementable approximations to these policies. These results and others are presented in a set of simulation studies that substantiates the basic idea and develops some of the intuitions required to design a particular broadcast program.

Hilbert R-tree: An Improved R-tree using Fractals

Abstract: We propose a new Rtree structure that outperforms all the older ones The heart of the idea is to facilitate the deferred splitting ap preach in R-trees This is done by proposing an ordering on the R-tree nodes This ordering has to be ‘good’, in the sense that it should group ‘similar’ data rectangles to gether, to minimize the area and perimeter of the resulting minimum bounding rectangles (MBRs) Following [KF93] we have chosen the so-called ‘2D-c’ method, which sorts rectangles according to the Hilbert value of the center of the rectangles Given the ordering, every node has a well-defined set of sibling nodes; thus, we can use deferred splitting By adjusting the split policy, the Hilbert R-tree can achieve as high utilization as desired To the contrary, the R-tree has no control over the space utilization, typically achieving up to 70% We designed the manipulation algorithms in detail, and we did a full implementation of the *This resexch was partially fuuded by the hmtitutc for Systerm, FLsemch (ISR), by the National Science Foundation under Grants IF&9205273 and IFU-8958546 (PYI), with matching fuuds from EMPRESS Software Inc und Think& h4achinen

Special finite element methods for a class of second order elliptic problems with rough coefficients

Abstract: In this paper the approximate solution of a class of second order elliptic equations with rough coefficients is considered. Problems of the type considered arise in the analysis of unidirectional composites, where the coefficients represent the properties of the material. Several methods for this class of problems are presented, and it is shown that they have the same accuracy as usual methods have for problems with smooth coefficients. The methods are referred to as special finite element methods because they are of finite element type but employ special shape functions, chosen to accurately model the unknown solution.

Vagal tone and the physiological regulation of emotion

Abstract: On the basis of current knowledge of neuroanatomy and our previous research with cardiac vagal tone, we have proposed the vagal circuit of emotion regulation. The vagal circuit of emotion regulation incorporates lateral brain function with the regulation of the peripheral autonomic nervous system in the expression of emotion. The vagus and the vagal circuit do not function independently of other neurophysiological and neuroendocrine systems. Research on brain activity (see Dawson, in this volume; Fox, in this volume) and research on adrenocortical activity (see Stansbury & Gunnar, in this volume) demonstrate that EEG and cortisol are related to emotion states and to individual differences similar to those that we have investigated. The vagal circuit emphasizes not only the vagus but also the lateralization of specific brain structures in emotion regulation. The emphasis of the vagal circuit on right-brain-stem structures stimulates several testable hypotheses regarding the function of specific structures in the right brain in emotion regulation. These speculations are consistent with other reports (see Dawson, in this volume; Fox, in this volume) describing asymmetrical EEG activity during expressed emotions. Moreover, the vagal circuit does not exist independently of the brain structures and peptide systems regulating cortisol (see Stansbury & Gunnar, in this volume). Areas in the brain stem regulating vagal activity are also sensitive to the peptides that regulate cortisol (e.g., vasopressin and corticotropin-releasing hormone). In this essay, we have provided information regarding the relation between vagal tone and emotion regulation. A review of research indicates that baseline levels of cardiac vagal tone and vagal tone reactivity abilities are associated with behavioral measures of reactivity, the expression of emotion, and self-regulation skills. Thus, we propose that cardiac vagal tone can serve as an index of emotion regulation. Historically, the vagus and other components of the parasympathetic nervous system have not been incorporated in theories of emotion. Recent developments in methodology have enabled us to define and accurately quantify cardiac vagal tone. Theories relating the parasympathetic nervous system to the expression and regulation of emotion are now being tested in several laboratories.

Employment schedules among dual-earner spouses and the division of household labor by gender.

Abstract: I extend the research on how employment schedules affect family life and which factors determine a husband's share in household labor I present a new specification of the concept of "available time" among dual-earner couples-the amount of overlap (in hours) in spouses' employment schedules and whether a spouse works day, evening, night, or rotating shifts. I show that variations in employment schedules are significant determinants of a husband's share in traditionally female household tasks. Also important are certain aspects of spouses' resources, gender role ideology, and stage in the life course. Interpretations of these findings take into account whether it is the husband's and/or wife's hours performing household tasks that are affected by these variables, since husband's share depends on both. The data come from the 1986-1987 National Survey of Families and Households, and my findings support the view that modest increases in husbands'participation in household labor may result from the growing diversity in employment schedules among American workers.

The TV-tree: an index structure for high-dimensional data

Abstract: We propose a file structure to index high-dimensionality data, which are typically points in some feature space. The idea is to use only a few of the features, using additional features only when the additional discriminatory power is absolutely necessary. We present in detail the design of our tree structure and the associated algorithms that handle such "varying length" feature vectors. Finally, we report simulation results, comparing the proposed structure with the R*-tree, which is one of the most successful methods for low-dimensionality spaces. The results illustrate the superiority of our method, which saves up to 80% in disk accesses.

Dynamic queries for visual information seeking

Abstract: Considers how dynamic queries allow users to "fly through" databases by adjusting widgets and viewing the animated results. In studies, users reacted to this approach with an enthusiasm more commonly associated with video games. Adoption requires research into retrieval and display algorithms and user-interface design. The author discusses how experts may benefit from visual interfaces because they will be able to formulate more complex queries and interpret intricate results. >

Personal and Structural Determinants of the Pace of Strategic Decision Making

Abstract: A model of the determinants of strategic decision-making pace that incorporates the role of individual differences among executive decision makers, organizational structural characteristics, and industry effects is developed. Drawing on data from 151 firms, we found that chief executive officers' cognitive ability, use of intuition, tolerance for risk, and propensity to act associated positively with speedy decisions. Decision pace appeared to be faster in centralized organizations and slower in formalized organizations. Our results also suggest that the construct of comprehensiveness has both cognitive and organizational structural aspects, with cognitive comprehensiveness relating positively and organizational comprehensiveness, negatively, to strategic decision-making pace.

A global 1° by 1° NDVI data set for climate studies. Part 2: The generation of global fields of terrestrial biophysical parameters from the NDVI

Abstract: A satellite-based 1° by 1° normalized difference vegetation index (NDVI) data set has been processed to derive land surface parameters for general circulation models of the atmosphere (GCMs). Prior to calculation of the land surface parameters, corrections were applied to the source NDVI data set to account for (i) obvious anomalies in the data time-series, (ii) the effect of variations in solar zenith angle, (iii) data dropouts in cold regions where a temperature threshold procedure designed to screen for clouds also eliminates cold land surface points, and (iv) persistent cloud cover in the tropics. An outline of the procedures for calculating land surface parameters from the corrected NDVI data set is given, and a brief description is provided of source material that was used in addition to the NDVI data. The data sets summarized in this paper should represent improvements over prescriptions currently used in land surface parameterizations in that the spatial and temporal dynamics of key land ...