1,099 research outputs found
A martingale analysis of first passage times of time-dependent Wiener diffusion models
Research in psychology and neuroscience has successfully modeled decision
making as a process of noisy evidence accumulation to a decision bound. While
there are several variants and implementations of this idea, the majority of
these models make use of a noisy accumulation between two absorbing boundaries.
A common assumption of these models is that decision parameters, e.g., the rate
of accumulation (drift rate), remain fixed over the course of a decision,
allowing the derivation of analytic formulas for the probabilities of hitting
the upper or lower decision threshold, and the mean decision time. There is
reason to believe, however, that many types of behavior would be better
described by a model in which the parameters were allowed to vary over the
course of the decision process.
In this paper, we use martingale theory to derive formulas for the mean
decision time, hitting probabilities, and first passage time (FPT) densities of
a Wiener process with time-varying drift between two time-varying absorbing
boundaries. This model was first studied by Ratcliff (1980) in the two-stage
form, and here we consider the same model for an arbitrary number of stages
(i.e. intervals of time during which parameters are constant). Our calculations
enable direct computation of mean decision times and hitting probabilities for
the associated multistage process. We also provide a review of how martingale
theory may be used to analyze similar models employing Wiener processes by
re-deriving some classical results. In concert with a variety of numerical
tools already available, the current derivations should encourage mathematical
analysis of more complex models of decision making with time-varying evidence
Measuring students\u27 perceptions of plagiarism: Modification and Rasch validation of a plagiarism attitude scale
Plagiarism is a significant area of concern in higher education, given university students\u27 high self-reported rates of plagiarism. However, research remains inconsistent in prevalence estimates and suggested precursors of plagiarism. This may be a function of the unclear psychometric properties of the measurement tools adopted. To investigate this, we modified an existing plagiarism scale (to broaden its scope), established its psychometric properties using traditional (EFA, Cronbach\u27s alpha) and modern (Rasch analysis) survey evaluation approaches, and examined results of well-functioning items. Results indicated that traditional and modern psychometric approaches differed in their recommendations. Further, responses indicated that although most respondents acknowledged the seriousness of plagiarism, these attitudes were neither unanimous nor consistent across the range of issues assessed. This study thus provides rigorous psychometric testing of a plagiarism attitude scale and baseline data from which to begin a discussion of contextual, personal, and external factors that influence students\u27 plagiarism attitudes
Segmentation of Images with Incompletely Specified Regions
Sometimes image regions are incompletely specified in
the sense that only a few representative points in each
region are known. In order to determine a segmentation of
such an image it is necessary to construct the region
boundaries. A fast algorithm called a diffusion algorithm
is given that determines region boundaries by diffusing
region labels outward from the known points
CONTEXTUAL BOUNDARY FORMATION BY ONE-DIMENSIONAL EDGE DETECTION AND SCAN LINE MATCHING
No abstract available
CONTEXTUAL BOUNDARY FORMATION BY SCAN LINE MATCHING
In this paper an algorithm is given for generating linked edge boundaries between adjacent regions of different
gray levels.
In contrast with peak following algorithms,
edges are treated as variable width regions, and the edge linking procedure is really a region grower.
Edge linking
is a parallel process on all the edges in pairs of adjacent scan lines, and contextual information in the direction of
the scan lines is used
to resolve ambiguous linking
situations. The procedure relies heavily
upon a one-
dimensional edge detector that defers the formation of local edge interpretations until more informed decisions can be made by the edge linking procedure
TEXTURE REGION GROWING BASED UPON A STRUCTURAL MODEL OF TEXTURE
In this paper the problem of texture boundary formation is
approached by a region growing technique that is based upon a structural
model of texture.
Region growing is based upon similarities among
texture elements and upon the spatial proximities of these elements. The region grower itself is a global algorithm that makes use of a minimal spanning tree of what is called the associated texture graph of
a texture. A primary advantage of the method is that the texture
regions are self-organizing in the sense that no artificial windows need
be superimposed over the source textures. While the region described in
this paper uses extrema-based texture elements, the application to other
types of elements is straightforward
Unsteady Flow and Whirl-Inducing Forces in Axial-Flow Compressors: Part I—Experiment
An experimental and theoretical investigation has been conducted to evaluate the effects seen in axial-flow compressors when the centerline of the rotor is displaced from the centerline of the static structure of the engine. This creates circumferentially nonuniform rotor-tip clearances, unsteady flow, and potentially increased clearances if the rotating and stationary parts come in contact. The result not only adversely affects compressor stall margin, pressure rise capability, and efficiency, but also generates an unsteady, destabilizing, aerodynamic force, called the Thomas/Alford force, which contributes significantly to rotor whirl instabilities in turbomachinery. Determining both the direction and magnitude of this force in compressors, relative to those in turbines, is especially important for the design of mechanically stable turbomachinery components. Part I of this two-part paper addresses these issues experimentally and Part II presents analyses from relevant computational models. Our results clearly show that the Thomas/Alford force can promote significant backward rotor whirl over much of the operating range of modern compressors, although some regions of zero and forward whirl were found near the design point. This is the first time that definitive measurements, coupled with compelling analyses, have been reported in the literature to resolve the long-standing disparity in findings concerning the direction and magnitude of whirl-inducing forces important in the design of modern axial-flow compressors
Case Control Study on Dialysis Arthropathy: The Influence of Two Different Dialysis Membranes: Data from the EDTA Registry
In a retrospective case control study the prevalence of signs and symptoms of dialysis osteoarthropathy was analysed. Cases and controls had received over 9 years of maintenance haemodialysis uninterrupted by peritoneal dialysis or transplantation. The cases comprised 55 patients treated predominantly with polyacrylonitrile (AN69) dialysers. They were compared to a matched group dialysed exclusively with cellulosic membranes. Over 60% of all patients, cases and controls, showed one or more signs of disabling osteoarthropathy, with joint pains occurring more frequently in the older age groups. Twenty-seven of the 55 cases who had received less than 2 years of cellulosic membrane dialysis followed by 7-12 years of AN69 dialysis tended to have a lower prevalence of joint pains, carpal-tunnel syndrome and bone cysts. However, no statistically significant differences were obtained compared to the matched control group dialysed exclusively on cellulosic membranes (mostly cuprophane). The remaining 28 cases, who had been treated for more than 2 years with cellulosic membranes preceding the longer treatment period with polyacrylonitrile dialysers, showed a prevalence similar to that of their cellulosic controls. This study thus shows little, if any, influence of the two types of membranes on the prevalence of signs and symptoms of β2-microglobulin amyloidosi
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