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排序方式: 共有1324条查询结果,搜索用时 10 毫秒
21.
Elizabeth J. Malloy Donna Spiegelman Ellen A. Eisen 《Computational statistics & data analysis》2009,53(7):2605-2616
This article presents an application and a simulation study of model fit criteria for selecting the optimal degree of smoothness for penalized splines in Cox models. The criteria considered were the Akaike information criterion, the corrected AIC, two formulations of the Bayesian information criterion, and a generalized cross-validation method. The estimated curves selected by the five methods were compared to each other in a study of rectal cancer mortality in autoworkers. In the stimulation study, we estimated the fit of the penalized spline models in six exposure-response scenarios, using the five model fit criteria. The methods were compared on the basis of a mean squared error score and the power and size of hypothesis tests for any effect and for detecting nonlinearity. All comparisons were made across a range in the total sample size and number of cases. 相似文献
22.
Increasingly powerful computers and increased emphasis on evidence based decision making are creating a demand for merging
and integrating data from different sources into a single data set. The demand for data is outstripping our ability to ensure
data integrity, and sometimes analysis is performed on data that are not appropriate for the purposes they are used for. Here
we describe problems that arise when data from different sources are merged, and we suggest that one way to add context to
data so that users can make informed decisions about their ontological context is through ontology-based metadata. Examples
of the problem are taken from health data with emphasis on difficulties in standardizing Emergency Room wait times. We describe
eight fields that can be used to capture contextual metadata. These fields are captured using ethnographic methods from users
and database stewards who frequently understand precisely how context and institutional usage have shaped interpretation of
semantic fields. We argue that attaching a portable archive of ontological context to travel with data—based on information
from users and developers—is a means of ensuring that data are integrated and compared in multiple contexts with greater integrity
and more robust results.
相似文献
Nadine SchuurmanEmail: |
23.
Faith Ellen Panagiota Fatourou Eleftherios Kosmas Alessia Milani Corentin Travers 《Distributed Computing》2016,29(4):251-277
A universal construction is a general mechanism for obtaining a concurrent implementation of an object from its sequential code. We show that there is no universal construction that is both disjoint-access parallel (guaranteeing the processes operating on different parts of an implemented object do not interfere with one another) and wait-free (guaranteeing progress for each nonfaulty process when accessing an object). In contrast, we present a universal construction which results in disjoint-access parallel, wait-free implementations of any object provided there is a bound on the number of data items accessed by each operation supported by the object. 相似文献
24.
In the cold, Purdue Pegboard (PP) performance declines. The purpose of this study was to determine if this cold-induced impairment is consistent across days (i.e. test-retest reliability) in 5°C. In thermoneutral air (25°C), 14 men were familiarised to the dominant hand (PPa) and bimanual (PPb) PP tasks. They then experienced two 90-min cold exposures (Day 1, Day 2) while wearing ~1 clo. Bare hands were maintained throughout. Performance on both tasks showed high reliability from day to day (intraclass correlations >0.700) in both thermoneutral and cold conditions. However for both tasks, room temperature performance did not predict performance in the cold (intraclass correlations <0.450). When screening applicants for manual labour in the cold, one must consider that room temperature dexterity does not correlate with dexterity in the cold. It is recommended that a 60-min period of cold exposure be employed to assess manual dexterity in these workers. STATEMENT OF RELEVANCE: This study shows that PP performance in room temperature does not predict performance in the cold but performance in the cold is consistent from day to day. When screening applicants for manual labour in the cold, it is recommended that dexterity tests be conducted in the same ambient conditions. 相似文献
25.
Optimization of Geoscience Laser Altimeter System waveform metrics to support vegetation measurements 总被引:1,自引:0,他引:1
The Geoscience Laser Altimeter System (GLAS) has collected over 250 million measurements of vegetation height over forests globally. Accurate vegetation heights can be determined using waveform metrics that include vertical extent and extent of the waveform's trailing and leading edges. All three indices are highly dependent upon the signal strength, background noise and signal-to-noise ratio of the waveform, as the background noise contribution to the waveforms has to be removed before their calculation. Over the last six years, GLAS has collected data during thirteen observation periods using illumination from three different lasers. The power levels of these lasers have changed over time, resulting in variable signal power and noise characteristics. Atmospheric conditions vary continuously, also influencing signal power and noise.To minimize these effects, we optimized a noise coefficient which could be constant or vary according to observation period or noise metric. This parameter is used with the mean and standard deviation of the background noise to determine a noise level threshold that is removed from each waveform. An optimization analysis was used with a global dataset of waveforms that are near-coincident with waveforms from other observation periods; the goal of the optimization was to minimize the difference in vertical extent between spatially overlapping GLAS observations. Optimizations based on absolute difference in height led to situations in which the total extent was minimized as well; further optimizations reduced a normalized difference in height extent. The simplest optimizations were based on a constant value to be applied to all observations; noise coefficients of 2.7, 3.2, 3.4 and 4.0 were determined for datasets consisting of global forests, global vegetation, forest in the legal Amazon basin and boreal forests respectively. Optimizations based on the power level or the signal-to-noise ratio of waveforms best minimized differences in waveform extent, decreasing the percent root mean squared height difference by 25-54% over the constant value approach. Further development of methods to ensure temporal consistency of waveform indices will be necessary to support long-term satellite lidar missions and will result in more accurate and precise estimates of canopy height. 相似文献
26.
Long-lived renaming allows processes to repeatedly get distinct names from a small name space and release these names. This
paper presents two long-lived renaming algorithms in which the name a process gets is bounded above by the number of processes
currently occupying a name or performing one of these operations. The first algorithm is asynchronous, uses LL/SC objects, and has step complexity that is linear in the number of processes, c, currently getting or releasing a name. The second is synchronous, uses registers and counters, and has step complexity that
is polylogarithmic in c. Both tolerate any number of process crashes. 相似文献
27.
28.
Marcia A. Mardis Ellen S. Hoffman Todd E. Marshall 《International Journal on Digital Libraries》2008,9(1):19-27
Despite their decade of deployment, educational digital libraries have not achieved sustained use in elementary and secondary
schools in the United States. Barriers to accessing the Internet and computers have been widely targeted by myriad initiatives,
but efforts aimed at bridging this first-level “digital divide” have not led to increased use of the Internet and digital
library resources in U.S. classrooms. In fact, such programs have revealed additional divides that affect educators’ use.
This paper examines the additional digital divide levels and proposes a new framework for understanding technology innovation
in schools that can improve development and outreach approaches by digital library developers. 相似文献
29.
Clara Mata Ellen K. Longmire David H. McKenna Katie K. Glass Allison Hubel 《Microfluidics and nanofluidics》2008,5(4):529-540
A recently proposed application of microfluidics is the post-thaw processing of biological cells. Numerical simulations suggest
that diffusion-based extraction of the cryoprotective agent dimethyl sulfoxide (DMSO) from blood cells is viable and more
efficient than centrifugation, the conventional method of DMSO removal. In order to validate the theoretical model used in
these simulations, a prototype was built and the flow of two parallel streams, a suspension of Jurkat cells containing DMSO
and a wash stream that contained neither cells nor DMSO, was characterized experimentally. DMSO transport in a rectangular
channel (depth 500 μm, width 25 mm and overall length 125 mm) was studied as a function of three dimensionless parameters:
depth ratio of the streams, cell volume fraction in the cell solution, and the Peclet number (Pe) based on channel depth, average flow rate and the diffusion coefficient for DMSO in water. In our studies, values of Pe ranged from O(103) to O(104). Laminar flow was ensured by keeping the Reynolds number between O(1) and O(10). Experimental results based on visual and
quantitative data demonstrate conclusively that a microfluidic device can effectively remove DMSO from liquid and cell laden
streams without compromising cell recovery. Also, flow conditions in the microfluidic device appear to have no adverse effect
on cell viability at the outlet. Further, the results demonstrate that we can predict the amount of DMSO removed from a given
device with the theoretical model mentioned previously. 相似文献
30.
Love John M.; Kisker Ellen Eliason; Ross Christine; Raikes Helen; Constantine Jill; Boller Kimberly; Brooks-Gunn Jeanne; Chazan-Cohen Rachel; Tarullo Louisa Banks; Brady-Smith Christy; Fuligni Allison Sidle; Schochet Peter Z.; Paulsell Diane; Vogel Cheri 《Canadian Metallurgical Quarterly》2005,41(6):885
Early Head Start, a federal program begun in 1995 for low-income pregnant women and families with infants and toddlers, was evaluated through a randomized trial of 3,001 families in 17 programs. Interviews with primary caregivers, child assessments, and observations of parent-child interactions were completed when children were 3 years old. Caregivers were diverse in race-ethnicity, language, and other characteristics. Regression-adjusted impact analyses showed that 3-year-old program children performed better than did control children in cognitive and language development, displayed higher emotional engagement of the parent and sustained attention with play objects, and were lower in aggressive behavior. Compared with controls, Early Head Start parents were more emotionally supportive, provided more language and learning stimulation, read to their children more, and spanked less. The strongest and most numerous impacts were for programs that offered a mix of home-visiting and center-based services and that fully implemented the performance standards early. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献