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At least 1,009 records · Page 56Linked to original sources
PubMed1994

A comparison of the performance of model-based confidence intervals when the correct model form is unknown: coverage of asymptotic means.

We conducted a simulation study to examine the performance of confidence intervals when multiplicative and additive rate (Poisson regression) models are fit to follow-up data, but the model form may be misspecified. Data were generated from over 129,000 different population structures that ranged from sub-additive to supra-multiplicative. When a multiplicative model was fit, all of the confidence intervals that we examined performed well as interval estimators of the asymptotic means of the point estimators, even when the correct model form was not multiplicative. When an additive model was fit, (1) only the likelihood ratio interval and the score interval with expected information consistently performed well, and they consistently performed better than any of the Wald intervals that we examined; (2) Wald intervals performed better when calculated with observed information rather than with expected information; and (3) Wald intervals with expected information performed better when the information matrix was evaluated at the restricted maximum likelihood estimate rather than the unrestricted maximum likelihood estimate.

Confidence Intervals↗
PubMed1994

[Dynamic modelling of the pathology-performance relationship].

Statistical modelling is insufficient to formalize the dynamic process of biological phenomena. This kind of model is also poor at explaining such processes. This theoretical position leads us to search for modelling techniques that allow the outcome to be transformed into (ideal) objects, substituting the intuition that underlies the observed signs with a specific and explicit mathematical construction. In this article, we present one example of such a model in ecopathology. This model is constructed with bibliographic data and permits us to measure the distance between a physical model based on maximization of a biomass (fattening of pigs), and a semantic model based on automatization of a system (porcine respiratory pathology).

Animals↗
PubMed2001

Information theoretical evaluation of parametric models of gain control in blowfly photoreceptor cells.

Models are developed and evaluated that are able to describe the response of blowfly photoreceptor cells to natural time series of intensities. Evaluation of the models is performed using an information theoretical technique that evaluates the performance of the models in terms of a coherence function and a derived coherence rate (in bit/s). Performance is gauged against a maximum expected coherence rate determined from the repeatability of the response to the same stimulus. The best model performs close to this maximum performance, and consists of a cascade of two divisive feedback loops followed by a static nonlinearity. The first feedback loop is fast, effectively compressing fast and large transients in the stimulus. The second feedback loop also contains slow components, and is responsible for slow adaptation in the photoreceptor in response to large steps in intensity. Any remaining peaks that would drive the photoreceptor out of its dynamic range are handled by the final compressive nonlinearity.

Adaptation, Ocular↗
PubMed2006

Impact of reactive settler models on simulated WWTP performance.

Including a reactive settler model in a wastewater treatment plant model allows representation of the biological reactions taking place in the sludge blanket in the settler, something that is neglected in many simulation studies. The idea of including a reactive settler model is investigated for an ASM1 case study. Simulations with a whole plant model including the non-reactive Takács settler model are used as a reference, and are compared to simulation results considering two reactive settler models. The first is a return sludge model block removing oxygen and a user-defined fraction of nitrate, combined with a non-reactive Takács settler. The second is a fully reactive ASM1 Takács settler model. Simulations with the ASM1 reactive settler model predicted a 15.3% and 7.4% improvement of the simulated N removal performance, for constant (steady-state) and dynamic influent conditions respectively. The oxygen/nitrate return sludge model block predicts a 10% improvement of N removal performance under dynamic conditions, and might be the better modelling option for ASM1 plants: it is computationally more efficient and it will not overrate the importance of decay processes in the settler.

Kinetics↗
PubMed2004

Work group diversity and group performance: an integrative model and research agenda.

Research on the relationship between work group diversity and performance has yielded inconsistent results. To address this problem, the authors propose the categorization-elaboration model (CEM), which reconceptualizes and integrates information/decision making and social categorization perspectives on work-group diversity and performance. The CEM incorporates mediator and moderator variables that typically have been ignored in diversity research and incorporates the view that information/decision making and social categorization processes interact such that intergroup biases flowing from social categorization disrupt the elaboration (in-depth processing) of task-relevant information and perspectives. In addition, the authors propose that attempts to link the positive and negative effects of diversity to specific types of diversity should be abandoned in favor of the assumption that all dimensions of diversity may have positive as well as negative effects. The ways in which these propositions may set the agenda for future research in diversity are discussed.

Cultural Diversity↗
PubMed2000

From simplicity to complexity: developing a model of practical skill performance in nursing.

The purpose of this article is to present and discuss a new model of practical skill performance in nursing. The model is conceptualized as having five components: substance and sequence; accuracy; fluency; integration; and caring conduct. The model challenges the truism of 'simple' nursing procedures. It is argued that performance of practical skills in nursing is characterized by complexity on many levels. Complexity lies within and between the components of the performance model and in the interaction between the nurse and the clinical context where practical nursing actions are performed. These complexities are described. Examples that illustrate the complex and reciprocal nature of these components are drawn from an empirical study of graduate nurses' development of practical skill in surgical hospital units. Implications of the model for education, practice and research are discussed.

Adult↗
PubMed1994

Comparison of the performance of two pulsed dye lasers using a synthetic stone model.

The performance of two pulsed dye laser lithotripters was compared in vitro using a synthetic calculus with gross physical properties similar to natural urinary tract calculi. Synthetic calculi were subjected to 500 shocks at nominal pulse energies of 40, 60, 80, 100 and 120 mJ using a 320 micron. fiber. Three calculi were treated at each energy level on the Technomed Pulsolith and two at each level on the Candela MDL 2000. The pattern of fragmentation was similar with both lithotripters. Increasing the energy level resulted in a linear increase in fragmentation (Pulsolith r = 0.95, MDL 2000 r = 0.99), but fragmentation was greater with the MDL 2000 than with the Pulsolith (6.01 mg/mJ versus 5.00 mg./mJ, p < 0.001). Both lasers fragmented the calculi satisfactorily. We recommend that all users of such lasers acquire a laser meter to allow monitoring of the energy delivered by the fiber tip.

Lithotripsy, Laser↗
PubMed2006

A new interpretation of ASM2d for modeling of SBR performance for enhanced biological phosphorus removal under different P/HAc ratios.

This study evaluated the prediction capability of Activated Sludge Model No. 2d (ASM2d), for the enhanced biological phosphorus removal (EBPR) performance of a sequencing batch reactor (SBR) receiving variable influent phosphate load. For this purpose, a laboratory-scale SBR was operated with a synthetic feed containing acetate as the sole carbon source. The experiments were conducted in four different Runs to ensure a range of different phosphate/acetate ratios in the influent. Model evaluations were carried out using concentration profiles measured throughout a representative cycle at steady state. An iterative calibration methodology was developed based on sensitivity analysis and applied to four different sets of experimental data on relevant model parameters reflecting SBR performance. ASM2d was able to predict the steady state behavior of the SBR system receiving variable influent phosphate loads only with the recalibrated parameter set. The regular changing pattern of the coefficients could be interpreted with the ability of the SBR system to sustain glycogen accumulating microorganisms, GAOs, which can store substrate under anaerobic conditions without polyphosphate energy, but deriving energy from the degradation of glycogen. Thus they are capable of prevailing at lower P/Ac ratios. The results indicate the need to include glycogen and GAOs as model components for processes involving both phosphate accumulating organisms, (PAOs) and GAOs, in order to obtain a better prediction of X(PHA) and oxygen uptake rate (OUR) profiles in the system.

Biodegradation, Environmental↗
PubMed2006

Performance of DFT in modeling electronic and structural properties of cobalamins.

Computational modeling of the enzymatic activity of B12-dependent enzymes requires a detailed understanding of the factors that influence the strength of the Co--C bond and the limits associated with a particular level of theory. To address this issue, a systematic analysis of the electronic and structural properties of coenzyme B12 models has been performed to establish the performance of three different functionals including B3LYP, BP86, and revPBE. In particular the cobalt-carbon bond dissociation energies, axial bond lengths, and selected stretching frequencies have been analyzed in detail. Current analysis shows that widely used B3LYP functional significantly underestimates the strength of the Co--C bond while the nonhybrid BP86 functional produces very consistent results in comparison to experimental data. To explain such different performance of these functionals molecular orbital analysis associated with axial bonds has been performed to show differences in axial bonding provided by hybrid and nonhybrid functionals.

Computer Simulation↗
PubMed2006

Error detection processes during observational learning.

The purpose of this experiment was to determine whether a faded knowledge of results (KR) frequency during observation of a model's performance enhanced error detection capabilities. During the observation phase, participants observed a model performing a timing task and received KR about the model's performance on each trial or on one of two trials. Delayed retention and transfer tests were used to assess the observer's ability to detect error in the model's performance and in the participant's performance while physically practicing the task. Results indicated a beneficial effect of a reduced KR frequency for performance stability and the ability to detect errors in both the model and the participant's own performance. The results suggest that aspects of the processing mechanism(s) developed in observational learning and related to KR are probably similar to those developed through physical practice.

Adolescent↗
PubMed2004

The performance of different models of primary care provision in Southern Africa.

Despite the emphasis placed during the last two decades on public delivery of comprehensive and equitable primary care (PC) to developing country populations, coverage remains far from universal and the quality often poor. Users frequently patronise private providers, ranging from informal drug sellers to trained professionals. Interest is increasing internationally in the potential for making better use of private providers, including contractual approaches. The research aim was to examine the performance of different models of PC provision, in order to identify their strengths and weaknesses from the perspective of a government wishing to develop an overall strategy for improving PC provision. Models evaluated were: (a) South African general practitioners (district surgeons) providing services under public contracts; (b) clinics provided in Lesotho under a sub-contract between a construction company and a South African health care company; (c) GP services provided through an Independent Practitioner Association to low income insured workers and families; (d) a private clinic chain serving low income insured and uninsured workers and their families; and (e) for comparative purposes, South African public clinics. Performance was analysed in terms of provider cost and quality (of infrastructure, treatment practices, acceptability to patients and communities), allowing for differences in services and case-mix. The diversity of the arrangements made direct comparisons difficult, however, clear differences were identified between the models and conclusions drawn on their relative performance and the influences upon performance. The study findings demonstrate that contextual features strongly influence provider performance, and that a crude public/private comparison is not helpful. Key issues in contract design likely to influence performance are highlighted. Finally, the study argues that there is a need before contracting out service provision to consider how the performance of private providers might change when the context within which they are working changes with the introduction of a contract.

Family Practice↗
PubMed2005

On-axis and off-axis viewing of images on CRT displays and LCDs: observer performance and vision model predictions.

RATIONALE AND OBJECTIVES: Although cathode-ray tube (CRT) displays typically are used for softcopy display of radiographs in the digital reading environment, liquid crystal displays (LCDs) currently are being used as an alternative. LCDs have many desirable viewing properties compared with a CRT, but significant image degradation can occur with off-axis viewing. This study compares observer performance and predictions from a human visual system model for on-axis and off-axis viewing for CRT displays versus LCDs. MATERIALS AND METHODS: A set of mammograms with different lesion contrasts (n = 400) were shown to six radiologists on CRT and LCD monitors, once on-axis and once off-axis. Observer performance was measured by using receiver operating characteristic techniques. Performance was correlated with results of a human vision model designed to predict observer performance (just noticeable difference [JND]metrix model). Two approaches were used to generate model metrics; a paired discrimination and channelized model observer approach. RESULTS: The performance of human observers indicated that on-axis viewing with the LCD was better than with the CRT, but off-axis viewing was significantly better with the CRT than LCD (F = 8.8175; P < .0001). The paired discrimination model correctly predicted on-axis, but not off-axis, results. The channelized model observer correctly predicted both on- and off-axis results. CONCLUSION: Off-axis viewing of radiographic images on an LCD monitor degrades human observer performance significantly compared with a CRT display. Care should be taken in the clinic to avoid off-axis viewing during diagnostic interpretation.

Breast Neoplasms↗
PubMed1992

Pacing lead survival: performance of different models.

UNLABELLED: Recent reports have shown poor survival of some leads currently in use. Long-term survival analysis of 2,444 leads (1,059 atrial and 1,385 ventricular) implanted in this institution since January 1980, and having at least 1 month of follow-up was performed. The survival of 123 different models was compared with the average survival of all the leads implanted in the corresponding chamber. Failure was defined as inactivation of the lead (electrical abandonment, explant, or cap) due to insulator and/or conductor fracture. RESULTS: The mean follow-up was 33 +/- 32 months. The cumulative survival for different atrial lead models was consistent with the average performance in the atrium. No atrial lead showed better or worse survival compared to the others. In the ventricular group, the Medtronic 4012 lead showed statistically significant poorer survival (P = 0.01) compared with the average survival of the ventricular leads. The Cardiac Pacemakers, Inc. (CPI) 4010 lead showed a nonsignificant (P = 0.12) worse performance than the average for ventricular leads. CONCLUSIONS: (1) The Medtronic 4012 had a significantly poorer performance than the rest of the leads. A trend in similar direction was found for the CPI 4010, also in the ventricular group; (2) Atrial lead models showed a stable survival; and (3) Frequent follow-up is required for some leads, especially in pacemaker dependent patients.

Actuarial Analysis↗
PubMed2006

Designing and performing experiments for model calibration using an automated iterative procedure.

Optimal experimental design for parameter estimation involves complex mathematical and practical steps in order to obtain a model with sufficiently accurate parameters. This paper proposes a methodology where user interaction is only required at the beginning of the experimental design procedure. All subsequent steps are carried out automatically, including: (1) finding the optimal experiment, (2) performing the experiments in practice, and (3) recalibrating the model. A software extension to an existing modelling and simulation package which performs this automatic procedure is also presented. Finally an illustration for the calibration of a one-step nitrification model using respirometric data is given. In this case, the application of the proposed procedure resulted, after three iterations, in considerably better confidence intervals on the parameter estimates, within the desired boundaries.

Automation↗
PubMed2004

The Rossby Centre Regional Atmospheric Climate Model part I: model climatology and performance for the present climate over Europe.

The Rossby Centre Atmospheric Regional Climate Model (RCA2) is described and simulation results, for the present climate over Europe, are evaluated against available observations. Systematic biases in the models mean climate and climate variability are documented and key parameterization weaknesses identified. The quality of near-surface parameters is investigated in some detail, particularly temperature, precipitation, the surface energy budget and cloud cover. The model simulates the recent, observed climate and variability with a high degree of realism. Compensating errors in the components of the surface radiation budget are highlighted and the fundamental causes of these biases are traced to the relevant aspects of the cloud, precipitation and radiation parameterizations. The model has a tendency to precipitate too frequently at small rates, this has a direct impact on the simulation of cloud-radiation interaction and surface temperatures. Great care must be taken in the use of observations to evaluate high resolution RCMs, when they are forced by analyzed boundary conditions. This is particularly true with respect to precipitation and cloudiness, where observational uncertainty is often larger than the RCM bias.

Atmosphere↗
PubMed2005

Robust classification modeling on microarray data using misclassification penalized posterior.

MOTIVATION: Genome-wide microarray data are often used in challenging classification problems of clinically relevant subtypes of human diseases. However, the identification of a parsimonious robust prediction model that performs consistently well on future independent data has not been successful due to the biased model selection from an extremely large number of candidate models during the classification model search and construction. Furthermore, common criteria of prediction model performance, such as classification error rates, do not provide a sensitive measure for evaluating performance of such astronomic competing models. Also, even though several different classification approaches have been utilized to tackle such classification problems, no direct comparison on these methods have been made. RESULTS: We introduce a novel measure for assessing the performance of a prediction model, the misclassification-penalized posterior (MiPP), the sum of the posterior classification probabilities penalized by the number of incorrectly classified samples. Using MiPP, we implement a forward step-wise cross-validated procedure to find our optimal prediction models with different numbers of features on a training set. Our final robust classification model and its dimension are determined based on a completely independent test dataset. This MiPP-based classification modeling approach enables us to identify the most parsimonious robust prediction models only with two or three features on well-known microarray datasets. These models show superior performance to other models in the literature that often have more than 40-100 features in their model construction. AVAILABILITY: Our MiPP software program is available at the Bioconductor website (http://www.bioconductor.org).

Algorithms↗
PubMed1985

Impact of environment on state university hospital performance. An explanatory model.

This article, by means of a path model, shows the impact of environment on state university hospital performance. The state environment is shown to affect systematically the intensity of local competition, the reputation of the medical school, the financial independence of the parent university, the allocation of indigent care dollars, the nature of governance and management, and the performance of the hospital. Results show that while certain environments are predictably more supportive of universities and university hospitals, these environments tend also to attract health care competition. Competition is shown to pressure hospitals into greater efficiency, but competition is shown also to hurt viability. But a hospital management that pays attention to the hospital environment is shown to help both efficiency and viability. The model explains respectively 37% and 46% of the variance in hospital efficiency and viability.

Data Collection↗