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Results for “Model performance”

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PubMed2005

Substantial effective sample sizes were required for external validation studies of predictive logistic regression models.

BACKGROUND AND OBJECTIVES: The performance of a prediction model is usually worse in external validation data compared to the development data. We aimed to determine at which effective sample sizes (i.e., number of events) relevant differences in model performance can be detected with adequate power. METHODS: We used a logistic regression model to predict the probability that residual masses of patients treated for metastatic testicular cancer contained only benign tissue. We performed standard power calculations and Monte Carlo simulations to estimate the numbers of events that are required to detect several types of model invalidity with 80% power at the 5% significance level. RESULTS: A validation sample with 111 events was required to detect that a model predicted too high probabilities, when predictions were on average 1.5 times too high on the odds scale. A decrease in discriminative ability of the model, indicated by a decrease in the c-statistic from 0.83 to 0.73, required 81 to 106 events, depending on the specific scenario. CONCLUSION: We suggest a minimum of 100 events and 100 nonevents for external validation samples. Specific hypotheses may, however, require substantially higher effective sample sizes to obtain adequate power.

Humans↗

PubMed1990

Eye-movement models for arithmetic and reading performance.

Three stochastic eye-movement models for arithmetic and reading performance have been proposed, one for arithmetic and two for reading. Each model characterizes a real-time stochastic process in terms of fixation durations and saccadic movement, but only direction and length of saccades are considered, not acceleration or velocity. Aspects of the models that are emphasized, partly because of their general neglect in the literature, are the probability distribution of fixation durations and the random walk of saccade directions. The distributions of fixation duration are approximately exponential, but systematic deviations can be accounted for in the models, even though the fit to data is not perfect. In the case of the arithmetic algorithms of addition and subtraction, the random walk of the normative model has only two possible moves. Data are also presented on backtracking, skipping and wandering eye movements, each of which has a significant relative frequency. The first reading model is called a minimal control model, because it does not take account of the effects of many local variables, e.g., word length, that have been extensively studied. The axioms on fixation duration for the minimal control model are the same as for the arithmetic model. Abstracting from the different arrangement of stimuli in arithmetic algorithms and in linear text, the axioms on saccadic motion for the two models are also essentially identical. The stochastic nature of both models is strongly supported by data on the independence of fixation durations from previous fixation durations. Additional detailed evidence is presented for the arithmetic model. To better account for a great variety of experimental results concerning significant effects on eye movements in reading, a text-dependent probabilistic model of reading is introduced. Significant local effects fall into three classes, identified as line, word and grammatical variables. The revised axioms embody five features of text known to be significant: (i) fixation duration depends on the number of letters in a word; (ii) a saccade is longer when a longer word is to the right; (iii) a saccade is longer when the current fixation is on a longer word; (iv) high-frequency fixation words have the highest probability of being skipped; (v) ambiguous or difficult grammatical structures increase backtracking.

Cognition↗

PubMed1996

Measuring rebreathing in spontaneous ventilation. A new lung model.

The performance of breathing systems is tested by means of physical lung models. We describe a new lung model, allowing direct measurement of the dead space/tidal volume ratios produced by any breathing system at different fresh gas flows. The model allows a range of different patterns of spontaneous respiration to be studied. Although rebreathing is measured by capnography, the design ensures that the results are unaffected by changes in carbon dioxide inflow, or even calibration drift in the capnograph.

Anesthesia, Closed-Circuit↗

PubMed1998

Effect of assisted circulation on left ventricular performance in a canine model.

BACKGROUND: Little is known about left ventricular performance during venoarterial bypass and left heart bypass (LHB) after cross-clamping the descending thoracic aorta. We evaluated the effects of venoarterial bypass and LHB on ventricular load optimization and left ventricular work efficiency. METHODS: We used the left ventricular conductance catheter and a micromanometer in 7 anesthetized mongrel dogs. We assessed preload by the end-diastolic volume, afterload by the effective arterial elastance, and left ventricular contractile properties by the slope of the end-systolic pressure-volume relationship. In addition, optimal ventricular arterial coupling (ratio of effective arterial elastance to slope of end-systolic pressure-volume relationship) and left ventricular work efficiency (ratio of external work to pressure-volume area) were calculated. RESULTS: The decrease in preload was much greater with LHB than venoarterial bypass. There were no significant differences in afterload and left ventricular contractility between venoarterial bypass and LHB. The ventricular arterial coupling during LHB was near 0.50 (0.69 +/- 0.16) in the "best heart" condition (effective arterial elastance = slope of end-systolic pressure-volume relationship/2), whereas the work efficiency during LHB was at maximum (0.73 +/- 0.12). CONCLUSIONS: We conclude that LHB has a more beneficial effect on left ventricular performance after cross-clamping of the descending thoracic aorta.

Animals↗

PubMed1991

A mathematical model of some aspects of jet nebuliser performance.

A simple mathematical model describing the performance of an Acorn jet nebuliser system (Medic-Aid Ltd) has been developed in which nebuliser efficiency E, defined as the volume fraction of solution (water) released as an aerosol or lost by evaporation, and nebulisation time T, are given as functions of the initial volume of solution. The model identifies an initial phase during which the nebuliser output is at a constant, continuous rate of 0.007 ml s-1. This is followed by an intermittent phase of operation during which output is estimated to occur for 0.25 of the total duration of the phase and results from solution deposited on the walls being recycled. The model indicates that increasing the flow of solution to the nebulisation region in the nebuliser or decreasing the fraction of aerosol intercepted by the baffle, will decrease T whilst leaving E unaffected. Two residence times tau 1, tau 2 which are a measure of the time that solution droplets adhere to the inner walls of the nebuliser are also identified; tau 1 (= 1.6 s) is the residence time associated with the rapid recycling of the majority (a fraction eta 1 = 0.992) of the aerosol while tau 2 (= 200 s) the residence time associated with the remaining fraction 1 - eta 1. Increasing eta 1 will increase E for constant tau 1, tau 2 while increasing tau 1, tau 2 will decrease E, T remaining essentially unchanged. It is proposed that the model may be applicable to other jet nebulisers.

Aerosols↗

PubMed1998

Herpes simplex virus encephalitis: cranial magnetic resonance imaging and neuropathology in a mouse model.

We performed a long-term magnetic resonance imaging (MRI) study in a mouse model of herpes simplex virus encephalitis. Mice were infected with herpes simplex virus type 1 (HSV-1) strain F. A 1.5-T cranial MRI scanner with standard spin-echo sequences was used. Neuropathological studies included immunohistochemistry. The presence of HSV DNA in brain tissue was determined with a polymerase chain reaction assay. Clinical assessment was performed daily: within the first 2 weeks the animals were severely affected and recovered thereafter. MRI and histopathological abnormalities corresponded well. HSV DNA was detectable initially and at 6 months. Extent and severity of structural abnormalities increased at 6 months. MRI offers a new in vivo approach for the detection of structural changes in the disease course of experimental herpes simplex virus encephalitis.

Animals↗

PubMed1999

Performance of a predictive model to identify undiagnosed diabetes in a health care setting.

OBJECTIVE: To develop a predictive model to identify individuals with an increased risk for undiagnosed diabetes, allowing for the availability of information within the health care system. RESEARCH DESIGN AND METHODS: A sample of participants from the Rotterdam Study (n = 1,016), aged 55-75 years, not known to have diabetes completed a questionnaire on diabetes-related symptoms and risk factors and underwent a glucose tolerance test. Predictive models were developed using stepwise logistic regression analyses with the absence or presence of newly diagnosed diabetes as the dependent variable and various items with a plausible connection to diabetes as the independent variables. The models were evaluated in another Dutch population-based study, the Hoorn Study (n = 2,364), in which the participants were aged 50-74 years. Performances of the predictive models were compared by using receiver-operator characteristics (ROC) curves. RESULTS: We developed three predictive models (PMs), PM1 contained information routinely collected by the general practitioner, while PM2 also contained variables obtainable by additional questions. The third predictive model, PM3, included variables that had to be obtained from a physical examination. These latter variables did not have additive predictive value, resulting in a PM3 similar to PM2. The area under the ROC curve was higher for PM2 than for PM1, but the 95% Cls overlapped (0.74 [0.70-0.78] and 0.68 [0.64-0.72], respectively). CONCLUSIONS: Using only information normally present in the files of a general practitioner, a predictive model was developed that performed similarly to one supplemented by information obtained from additional questions. The simplicity of PM1 makes it easy to implement in the current health care setting.

Aged↗

PubMed2004

Anomalous critical exponents in the anisotropic Ashkin-Teller model.

We perform a rigorous computation of the specific heat of the Ashkin-Teller model in the case of small interaction and we explain how the universality-nonuniversality crossover is realized when the isotropic limit is reached. We prove that, even in the region where universality for the specific heat holds, anomalous critical exponents appear: for instance, we predict the existence of a previously unknown anomalous exponent, continuously varying with the strength of the interaction, describing how the difference between the critical temperatures rescales with the anisotropy parameter.

Journal Article↗

PubMedDate not supplied

Schematic eye models for simulation of patient visual performance.

PURPOSE: To determine if model eyes can simulate the visual performance of normal human eyes under conditions of varying low myopic blur, pupil size, and contrast. METHODS: High and low contrast Bailey-Lovie logMAR visual acuity (VA) of three normal eyes of three subjects were measured for four artificial pupil sizes and ten levels of myopic defocus. Simulated visual acuities were then determined for three model eyes--the Indiana Eye with no spherical aberration, the Indiana Eye with average spherical aberration, and the Kooijman Eye--by generating optically aberrated VA charts for each testing condition using Visual Optics Lab software by Sarver and Associates, Inc, and having the subjects read high resolution printouts of these charts using a 3-mm pupil and optimal spectacle correction. The correlation between real VA and simulated VA was then plotted and a regression line calculated. RESULTS: Slopes for the Indiana Eye, Indiana Eye with spherical aberration, and Kooijman Eye were 0.98, 0.98, and 1.01 for high contrast, and 0.92, 0.67, and 0.75 for low contrast, respectively. The r2 values were 0.73, 0.74, and 0.77, for high contrast, and 0.69, 0.40, and 0.50 for low contrast, respectively. Under low contrast conditions the Indiana Eye VA was significantly closer to the real VA than that of the other two models (P<.0003). CONCLUSION: Visual performance can be simulated by eye models. The simple single surface Indiana Eye with no spherical aberration best modeled both high and low contrast visual acuity.

Adult↗

PubMed2005

Prehospital prediction of intensive care unit stay and mortality in blunt trauma patients.

BACKGROUND: The success of a trauma system relies on transfer of patients from the field to the most appropriate hospital for definitive care. However, no consensus has been reached regarding the best criteria or triage tool for identifying patients injured seriously enough to warrant transfer to a trauma center. METHODS: Predictors of mortality and intensive care unit stay were identified and prediction models developed in a design data set. The performance of these models was evaluated in a test data set and compared with current trauma triage guidelines, derived from the American College of Surgeons model. RESULTS: The newly developed prediction models performed comparably with the current trauma triage guidelines. CONCLUSION: Although the performance of newly developed triage models was promising, their performance did not exceed that of the current trauma triage guidelines. In particular, the anatomic injury criteria appeared to be the key component of the current trauma triage guidelines.

Decision Support Techniques↗

PubMed1984

Epikeratophakia: clinical evaluation and histopathology of a non-human primate model.

We performed four aphakic and two myopic epikeratophakia procedures in a non-human primate model using two fresh and three prefrozen corneas and one glycerin-preserved cornea. The experiments were terminated 4 to 5 months postoperatively, and the eyes examined with correlative microscopy. The glycerin-preserved cornea sloughed and was not available for analysis. All but one prefrozen specimen demonstrated donor keratocyte repopulation. Active keratocytes were noted lining the optical interface. Focal areas of missing Bowman's layer were present in three recipient corneas. Epithelial interface ingrowth occurred in two specimens. The recipient corneas were otherwise normal.

Animals↗

PubMed2000

The influence of shared mental models on team process and performance.

The influence of teammates' shared mental models on team processes and performance was tested using 56 undergraduate dyads who "flew" a series of missions on a personal-computer-based flight-combat simulation. The authors both conceptually and empirically distinguished between teammates' task- and team-based mental models and indexed their convergence or "sharedness" using individually completed paired-comparisons matrices analyzed using a network-based algorithm. The results illustrated that both shared-team- and task-based mental models related positively to subsequent team process and performance. Furthermore, team processes fully mediated the relationship between mental model convergence and team effectiveness. Results are discussed in terms of the role of shared cognitions in team effectiveness and the applicability of different interventions designed to achieve such convergence.

Achievement↗

PubMed2006

Evaluation of variance models for fractionator sampling of trees.

We compared the performance of several models for predicting, from small samples, the precision of estimates of the total number of blossoms on fruit trees obtained using a three-stage fractionator, in which the sampling units were defined by the tree structure: (1) primary branches and stem (2) secondary branches and shoots and (3) flowering buds. The models considered were the semiempirical models of Cruz-Orive (1990, 2004) (CO), a random sample model (SR), a Poisson model (P), successive differences (D) and repeated systematic sampling (R). Procedures that relied upon a single sample and a model of the variance (SR, P, D) were not able to predict the estimator variance because the considered structures strongly violated model assumptions. The semiempirical CO model performed acceptably in some cases where model assumptions were violated to a moderate degree. The repeated systematic sampling procedure, which does not rely upon a model of the variance, usually provided very good predictions when the resampling terms were distributed appropriately across more than one sampling stage.

Flowers↗

PubMedDate not supplied

Demonstrating theory in practice: examples of the McGill model of nursing.

An important test for a model of nursing is its usefulness in practice. One-hundred and forty-seven nurses in one hospital attended a 2-day workshop on the McGill Model of Nursing. An evaluation of this workshop showed the usefulness of the McGill model. Self-efficacy for performing the model increased, as did behaviors and performance related to nursing by this model. Self-reports and supervisor reports measured these increases 6 months after the workshop. Further, the implementation of the model led to staff nurses initiating bottom-up changes in the nursing department. Boundary conditions or factors outside of the workshop content that enhanced the adoptation of the model are described.

Adaptation, Psychological↗

PubMed2000

Air distribution during in situ air sparging: an overview of mathematical modeling.

The performance of in situ air sparging is controlled by the distribution of air pathways in the subsurface, which is in turn controlled by the structure of the medium to be sparged. The specific pathways that the air follows are determined, at the grain scale, by the distribution of air entry pressures of the pores. At the field scale, pore size distributions are usually correlated with heterogeneous structures (e.g. layers) within the medium, which control the macroscopic distribution of the air. The processes that produce an observed air distribution at a particular site are complicated, and are potentially well suited to modeling with multiphase flow models. Recent numerical modeling of heterogeneous media appears to successfully represent expected distributions of air; however, current models do not provide a tool to predict sparging performance. For this to be the case, the models need to represent the detailed structure of the medium at the site to be studied, as well as to capture the relevant aspects of the discrete air/water distribution (i.e., represent air channels at the centimeter or smaller scale). This will, in general, require a level of site data that is not available and numerical models that require many millions of computational elements. As a consequence, at least for the foreseeable future, numerical modeling of the air sparging process will continue to play a vital role as a conceptual tool with limited predictive capability at sites.

Air↗

PubMed1995

Artificial neural networks simulating visual texture segmentation and target detection in line-element images.

Measurements were made of human observers' performance both in segmenting regions of line-elements and in detecting line-element targets in stimuli containing several orientations. Performance was modelled by four artificial neural networks constructed from processing units trained to mimic the gross functionality of certain loosely defined classes of cortical cells. Model 1 contained modules sensitive to absolute orientation only, and it provided a poor fit to the human-performance data. Model 2 contained modules sensitive to orientation contrast: the outputs of these modules could be suppressed with fields of uniformly oriented line-elements. Model 3 contained orientation-contrast-sensitive modules of a different type: their outputs could be suppressed with fields of randomly oriented line-elements. Models 2 and 3 both successfully processed line-element arrays with orientation heterogeneities, but these models still provided inadequate fits to the human-performance data. Model 4 contained both types of orientation-contrast-sensitive modules; this model was able to account for human performance in the segmentation and detection tasks, both qualitatively and quantitatively.

Contrast Sensitivity↗

PubMed2001

Roles of native topology and chain-length scaling in protein folding: a simulation study with a Go-like model.

We perform folding simulations on 18 small proteins with using a simple Go-like protein model and analyze the folding rate constants, characteristics of the transition state ensemble, and those of the denatured states in terms of native topology and chain length. Near the folding transition temperature, the folding rate k(F) scales as k(F) approximately exp(-c RCO N(0.6)) where RCO and N are the relative contact order and number of residues, respectively. Here the topology RCO dependence of the rates is close to that found experimentally (k(F) approximately exp(-c RCO)), while the chain length N dependence is in harmony with the predicted scaling property (k(F) approximately exp(-c N(2/3))). Thus, this may provides a unified scaling law in folding rates at the transition temperature, k(F) approximately exp(-c RCO N(2/3)). The degree of residual structure in the denatured state is highly correlated with RCO, namely, proteins with smaller RCO tend to have more ordered structure in the denatured state. This is consistent with the observation that many helical proteins such as myoglobin and protein A, have partial helices, in the denatured states. The characteristics of the transition state ensemble calculated by the current model, which uses native topology but not sequence specific information, are consistent with experimental phi-value data for about half of proteins.

Algorithms↗

PubMed2006

Performance of the partition-limited model on predicting ryegrass uptake of polycyclic aromatic hydrocarbons.

Accurate modeling of the uptake and accumulation behavior of organic contaminants like polycyclic aromatic hydrocarbons (PAHs) in plants is essential to assess crop contamination and subsequent human exposure. In this study, the performance of a partition-limited model on predicting ryegrass uptake of PAHs (acenaphthene, fluorene, phenanthrene and pyrene) from water was evaluated and the major factors were examined. It was found that model predictions of PAH concentrations in roots and shoots of ryegrass were all within an order of magnitude of the observed values with the differences between estimated and measured concentrations less than 42.1% for roots and 78.4% for shoots. Since the model considered soil/water-plant pathway only, it was inevitable that simulated concentrations in shoots suffered a bigger error than those in roots due to the influence of foliar uptake, the other important pathway for PAHs. If the impact of foliar uptake was excluded, the accuracy of simulated shoot concentrations would be greatly enhanced, with the maximum prediction error reduced from 78.4% to 47.1% for pyrene. Other factors aside from foliar uptake were also examined, including aqueous PAH concentrations, uptake time, plant composition and chemical properties. These factors were found to influence the model performance generally through acting on the quasi-equilibrium factor (alpha(pt)). Results from this study substantiated the utility of this partition-limited model for vegetation-uptake assessment, and then provided some testimony valuable for the modification of a model with a better performance.

Lolium↗