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PubMed1994

The management of information: storage and retrieval of data.

Internationally harmonized and cost-effective control of chemicals marketed worldwide greatly depend both on the generation of and easy access to reliable and comparable experimental information. Stored data are of use only if information can be retrieved quickly in an understandable form. Some models and theories of information retrieval (e.g. fuzzy set theory, probabilistic approach, artificial intelligence) are briefly discussed first, then followed by applications (such as indexing and clustering techniques). Finally the structure of databases is briefly reviewed.

Database Management Systems↗
PubMed1993

Evaluating the quality of a probabilistic diagnostic system using different inferencing strategies.

In this paper we describe the evaluation of a probabilistic diagnostic system for patients with renal mass. Three inference models: Multi-membership Bayesian (MB), Minimal Diagnosis (MD) and Bayesian Network (BN), and 72 patients are used to illustrate three interrelated measures of system performance: accuracy, reliability and discriminating power. The inferencing strategies we tested demonstrated the kind of trade-offs in the performance measures that can be expected from imperfect systems. Ultimately, the purpose and expected use of a system should dictate the relative importance ascribed to different aspects of system performance.

Adolescent↗
PubMed1990

[Moore's model for systemic classification of biological phenomena].

The common language of many researchers of different scientific fields, is the systematic method. In this paper the concept of abstract systems is developed. The applications of multidimensional abstract systems, of oriented probabilistic type, in biological systems is especially stressed. For mathematical interpretation of the problem, Moore's model is proposed.

Models, Biological↗
PubMed1998

Counting and discounting gained life-years.

The life expectancy gain produced by a reduction in mortality can be determined by three different methods with respect to the timing of the gained life-years. One method adds the life expectancy gain to the expected end of life. Another method places the gain at the time of occurrence of the mortality reduction. A third method distributes the gained life-years over the maximum lifespan according to the differences in survival probabilities after and before the reduction in mortality. The three methods are all used in the literature together with a quasi-deterministic and a probabilistic approach to the notion of life expectancy. The counted numbers of gained life-years are the same, but due to different timing of life expectancy gains the discounted numbers are different. Several discounting models are identified when combining the three methods of counting with the deterministic and the probabilistic approaches to life expectancy. Some are symmetrical, some are not. However, most importantly, they come out with potentially very large differences in the discounted number of gained life-years. They differ by a factor of approximately (1 + r)e(a)-1, where r is a constant discount rate and e(a) is remaining life expectancy at age a, when the reduction of mortality occurs. For a new-born, discounting at 7% p.a., one discounting model provides a present value that is 150 times larger than another discounting model, the other models being in between. The various counting and discounting models for life expectancy gains are presented formally, graphically, and with numerical examples using Danish male mortality data. We show how three different discounting models provide large differences in discounted life expectancy gains and hence cost-effectiveness ratios in an economic evaluation of a colorectal cancer screening programme in Denmark. These different discounting models co-exist in the evaluation literature. Choice of method is rarely made explicit. Sensitivity analysis with respect to this choice is even rarer. We argue that one counting-discounting model is sufficient and that this should be to discount the differences between the two survival probability curves.

Adult↗
PubMed2005

A study of hominin dispersal out of Africa using computer simulations.

A study of hominin dispersal out of Africa using computer simulations is presented. Attention is focused on the joint probability of the colonization of Western Europe later than 1 Ma and that of Eastern Asia prior to 1.6 or 1.8 Ma, as current archaeological estimates suggest. We found that the determinant factor to hominin dispersal is the mode of hominin movement. If the movement of all populations is uniform and their number great enough, greater than 300 in our models, then such movement favors the colonization of Eastern Asia and Western Europe at more or less the same time. On the other hand, the colonization acquires prominent probabilistic features if the number of populations migrating is small enough, smaller than 10 in our models, or when all hominin populations may move but there are only a few with much higher mobility. In this case, the joint probability for the earliest dispersals of hominins in Western Europe after 1 Ma and Eastern Asia prior to 1.6 Ma ranges from 0.02 to 0.05. The single probability of colonization of Western Europe after 1 Ma is very high, about 0.5 for the majority of the colonization routes, whereas the corresponding probability of the colonization of Eastern Asia prior to 1.6 Ma is ten times lower, about 0.05. The least probable event is the earliest colonization of Java prior to 1.6 Ma, to which our simulation attributes a probability of ca 0.01. Deserts, mountains, and mountain ranges may delay the arrival at a certain location; nevertheless, their effect on the joint probability is very small.

Africa↗
PubMedDate not supplied

Modelling relative recency discrimination tasks using a stochastic working memory model.

Relative recency discrimination (RD) task is typically used to assess the temporal organization function of the prefrontal cortex (PFC). Subjects look at a series of cards (with words or drawings on them) and on seeing a test card determine which of the two items was seen more recently. Results show that patients with damage to the prefrontal cortex are severely impaired on this task. We propose a memory trace-priming mechanism, based on automatic time-marking process hypothesis (Schacter, D.L., 1987. Memory, amnesia, and frontal lobe dysfunction: a critique and interpretation. Psychobiology 15, 21-36), to offer a computational account of the results. In this model, successive words seen by subjects leave decaying memory traces in PFC, which subsequently prime the representations in higher sensory areas such as inferior temporal Cortex (IT) during discrimination judgements. The paper focuses on the evaluation of a probabilistic pre-frontal trace mechanism using a pool of clusters of neurons with self-sustained firing that ends at a random time. The results show that the probabilistic behavior of subjects can be accounted for by the stochasticity of the trace model. A good fit to experimental data is obtained with a PFC memory persistence probability with a decay time constant of tau = approximately 30 s. The model allows for a distributed representation in IT and PFC, but the best fit suggests a sparse representation. It is concluded that further data are needed on representations in IT and PFC, on the connectivity between these two areas, and on the statistical and dynamic properties of memory neurons in PFC.

Discrimination, Psychological↗
PubMed2006

Ensemble forecasting of species distributions.

Concern over implications of climate change for biodiversity has led to the use of bioclimatic models to forecast the range shifts of species under future climate-change scenarios. Recent studies have demonstrated that projections by alternative models can be so variable as to compromise their usefulness for guiding policy decisions. Here, we advocate the use of multiple models within an ensemble forecasting framework and describe alternative approaches to the analysis of bioclimatic ensembles, including bounding box, consensus and probabilistic techniques. We argue that, although improved accuracy can be delivered through the traditional tasks of trying to build better models with improved data, more robust forecasts can also be achieved if ensemble forecasts are produced and analysed appropriately.

Ecosystem↗
PubMed2006

A method of assessing ecological risk to night heron, Nycticorax nycticorax, population persistence from dichlorodiphenyltrichloroethane exposure.

This paper first introduces a probabilistic method for quantitatively evaluating the effects of chemical pollutants in the environment on a wildlife population, which was applied to assess the ecological risk to night heron (Nycticorax nycticorax) population persistence from dichlorodiphenyltrichloroethane (DDE) exposure in Tai Lake, China. Intrinsic rate of population increase (r) calculated with a population age-structured matrix model was used to measure the adverse effect on population. To perform a probabilistic analysis of risk, lost intrinsic rate of population increase (deltaZ) because of DDE exposure was applied to express the exact extent of risk. The result showed that the risk (i.e., the expectancy of deltaZ of the night heron population exposed to DDE in Tai Lake) was 0.0259, indicating a decrease in gross population size of 2.56% every year compared with that of the previous year without DDE exposure.

Animals↗
PubMed2006

A cost-utility analysis of pimecrolimus vs. topical corticosteroids and emollients for the treatment of mild and moderate atopic eczema.

BACKGROUND: Conventional treatments for atopic eczema include topical corticosteroids (TCS) and emollients. Pimecrolimus, an immunosuppressant, was licensed in the U.K. in 2003 as an alternative treatment of mild to moderate atopic eczema. OBJECTIVES: To assess the cost-utility of pimecrolimus as a treatment for mild and moderate atopic eczema when compared with conventional treatments which use TCS and emollients. METHODS: A Markov state-transition model was developed to represent the cyclical nature of atopic eczema and provide an economic analysis of cost-utility for treatment alternatives from the perspective of a third party payer (U.K. National Health Service). A range of methods was used to obtain data for transition probabilities, costs and quality of life. These included a systematic review of published effectiveness data, expert opinion, and a utility study conducted by the authors. Separate cohort analyses were modelled to distinguish between children and adult populations and between differing treatment patterns for facial and body eczema. One-way sensitivity analyses and probabilistic sensitivity analysis (using Monte-Carlo simulation) were performed. RESULTS: Baseline cost-utility outputs from the model show that, in all tested scenarios, TCS dominate pimecrolimus (i.e. TCS are both cheaper and more effective). However, the differences in benefits between treatments output by the model are very small. Sensitivity analyses highlight the importance of cost variations in pimecrolimus. Where pimecrolimus is compared with emollient only it is probably cost effective at a willingness-to-pay threshold of 30 000 UK pounds per quality-adjusted life year. CONCLUSIONS: There are likely to be few situations in which the use of pimecrolimus for the treatment of atopic eczema can be justified on economic grounds. Exceptions are likely to be in cases where TCS have been shown to be ineffective, unacceptable due to adverse events, or where a patient is unwilling to accept TCS treatment despite appropriate education and support and emollient alone is the alternative clinical option.

Adult↗
PubMed2005

Whole cell stochastic model reproduces the irregularities found in the membrane potential of bursting neurons.

Irregular intrinsic behavior of neurons seems ubiquitous in the nervous system. Even in circuits specialized to provide periodic and reliable patterns to control the repetitive activity of muscles, such as the pyloric central pattern generator (CPG) of the crustacean stomatogastric ganglion (STG), many bursting motor neurons present irregular activity when deprived from synaptic inputs. Moreover, many authors attribute to these irregularities the role of providing flexibility and adaptation capabilities to oscillatory neural networks such as CPGs. These irregular behaviors, related to nonlinear and chaotic properties of the cells, pose serious challenges to developing deterministic Hodgkin-Huxley-type (HH-type) conductance models. Only a few deterministic HH-type models based on experimental conductance values were able to show such nonlinear properties, but most of these models are based on slow oscillatory dynamics of the cytosolic calcium concentration that were never found experimentally in STG neurons. Based on an up-to-date single-compartment deterministic HH-type model of a STG neuron, we developed a stochastic HH-type model based on the microscopic Markovian states that an ion channel can achieve. We used tools from nonlinear analysis to show that the stochastic model is able to express the same kind of irregularities, sensitivity to initial conditions, and low dimensional dynamics found in the neurons isolated from the STG. Without including any nonrealistic dynamics in our whole cell stochastic model, we show that the nontrivial dynamics of the membrane potential naturally emerge from the interplay between the microscopic probabilistic character of the ion channels and the nonlinear interactions among these elements. Moreover, the experimental irregular behavior is reproduced by the stochastic model for the same parameters for which the membrane potential of the original deterministic model exhibits periodic oscillations.

Action Potentials↗
PubMed1990

Genesis of sigmoidal dose-response curve during defibrillation by random shock: a theoretical model based on experimental evidence for a vulnerable window during ventricular fibrillation.

The sigmoidal dose-response curve (percent success vs shock energy) suggests a probabilistic nature of defibrillation. The mechanism is still largely unknown, however, random variation in the excitable state during ventricular fibrillation (VF) is suspected. A canine defibrillation study was designed to determine whether random variation in absolute VF voltage (AVFV) (a crude marker of number of excitable cells) was related to success of defibrillation, using a DC shock successful at the 50% level. The results were: (a) transmyocardial resistance (73.4 +/- 1.4 vs 73.6 +/- 1.5 ohms) and delivered energy (6.1 +/- 1.2 vs 6.2 +/- 1.2 joules) were similar; however, (b) AVFV 2 msec prior to DC shock was greater for successful as compared to unsuccessful attempts (0.5 +/- 0.1 vs 0.3 +/- 0.0 mV, P less than 0.01). A mathematical model was subsequently developed based on fluctuation in the number of excitable cells. Variation in the state of excitability resulted in a cyclic window potentially vulnerable to defibrillation. The vulnerable window occurred at a point when the number of excitable cells was low, i.e., a higher state of total depolarization, which was in agreement with the experimental finding. For a given VF pattern, duration of the vulnerable window was regulated by the shock energy. A larger shock energy generated a wider vulnerable window and, in turn, a higher success rate. Finally, the sigmoidal dose-response curve of defibrillation was theoretically constructed by calculating the variable chances of a random DC shock occurring either in a vulnerable window or elsewhere during VF. It is concluded that a vulnerable window susceptible to defibrillation can be demonstrated in the early stages (10 sec) of VF. The mathematical model provides a theoretical basis for the vulnerable window and helps elucidate the probabilistic nature of defibrillation.

Animals↗
PubMed2001

Refinement of the concept of uncertainty.

AIMS: To analyse the conceptual maturity of uncertainty; to develop an expanded theoretical definition of uncertainty; to advance the concept using methods of concept refinement; and to analyse congruency with the conceptualization of uncertainty presented in the theory of hope, enduring, and suffering. BACKGROUND: Uncertainty is of concern in nursing as people experience complex life events surrounding health. In an earlier nursing study that linked the concepts of hope, enduring, and suffering into a single theoretical scheme, a state best described as 'uncertainty' arose. This study was undertaken to explore how this conceptualization fit with the scientific literature on uncertainty and to refine the concept. DESIGN/METHODS: Initially, a concept analysis using advanced methods described by Morse, Hupcey, Mitcham and colleagues was completed. The concept was determined to be partially mature. A theoretical definition was derived and techniques of concept refinement using the literature as data were applied. FINDINGS: The refined concept was found to be congruent with the concept of uncertainty that had emerged in the model of hope, enduring and suffering. CONCLUSIONS: Further investigation is needed to explore the extent of probabilistic reasoning and the effects of confidence and control on feelings of uncertainty and certainty.

Adaptation, Psychological↗
PubMed1993

Recognising psychiatric symptoms. Relevance to the diagnostic process.

Current overemphasis on nosological diagnosis has led to a neglect of the process of symptom recognition. There is evidence, however, that the perception of the symptom alone does not guarantee symptom ascertainment since a decision-making component is also involved. To achieve the latter, additional information must be provided by the contextual cues implicit in the ongoing diagnostic hypothesis. Current diagnostic systems, however, still assume a two-stage model according to which symptom and disease recognition are independent cognitive events. This paper suggests that this model is inadequate and that descriptive psychopathology is not transparent. It then describes a neural network simulation to make various aspects of the problem explicit. This takes into account the multidimensional and probabilistic aspects of symptom recognition and is, from this point of view, superior to traditional algorithmic models. It also has the capacity to represent the different cognitive styles involved in symptom recognition.

Decision Support Techniques↗
PubMed2006

Decompression schedule optimization with an isoprobabilistic risk of decompression sickness.

INTRODUCTION: Divers use decompression schedules to reduce the probability of occurrence of decompression sickness when returning to the surface at the end of a dive. The probability of decompression sickness resulting from these schedules varies across different dives and the models used to generate them. Usually the diver is unaware of this variance in risk. This paper describes an investigation into the feasibility of producing optimized iso-probabilistic decompression schedules that minimize the time it takes for a diver to reach the surface. METHODS: The decompression schedules were optimized using the sequential quadratic programming method (SQP), which minimizes the ascent time for a given probability of decompression sickness. The U.S. linear-exponential multi-gas model was used to calculate an estimate of the probability of decompression sickness for a given dive. In particular 1.3-bar oxygen in helium rebreather bounce dives to between 18 m and 81 m were considered and compared against the UK Navy QinetiQ 90 tables for a similar estimate of probability of decompression sickness. RESULTS: The SQP method reliably produced schedules with fast and stable convergence to an optimized solution. Comparison of the optimized decompression schedules with the QinetiQ 90 schedules showed similar stop times for shallow dives to 18 m. For dives with a maximum depth of 39 m to 81 m, optimizing the decompression resulted in savings in decompression time of up to 30 min. CONCLUSIONS: This paper has shown that it is feasible to produce optimized iso-probabilistic decompression tables given a reliable risk model for decompression sickness and appropriate dive trials.

Decompression↗
PubMed2002

Crystal structure solution of proteins by direct methods: an automatic procedure for SIR-MIR and SIRAS-MIRAS cases.

In the previous paper [Giacovazzo & Siliqi (2002). Acta Cryst. A58, 590-597], a probabilistic approach for the SIR-MIR and the SIRAS-MIRAS cases has been described. The mathematical technique is able to take into account the errors arising from measurements, lack of isomorphism and heavy-atom model substructure. An automatic procedure is here described in which the conclusive formulas of that probabilistic approach have been implemented. The procedure has been successfully applied to several test structures: it can automatically provide, starting from the experimental data, high-quality electron-density maps.

Journal Article↗
PubMed2006

Evaluating the effectiveness of the Massachusetts workforce development system using no-shows as a nonexperimental comparison group.

This article examines the effect of the Massachusetts workforce development system on the earnings of disadvantaged adults using nonexperimental data from the late 1990s. The authors construct a comparison sample for program participants using individuals who apply for and are offered services yet do not participate in a training program. They present a series of difference-in-difference estimates that make several alternative efforts to correct for selectivity bias, including econometric models that regression adjust for observable characteristics and fixed-effect models that adjust for time-invariant person effects. They also employ probabilistic matching techniques to more finely align the treatment and comparison samples. On average, program participants experienced 20% increases in annual earnings 1 year postintervention and 25% increases after 2 years. The authors uncover considerable heterogeneity in these effects, suggesting that the most difficult to serve and the most job ready benefit the least.

Education↗
PubMed2003

Predicting the genotoxicity of polycyclic aromatic compounds from molecular structure with different classifiers.

Classification models were developed to provide accurate prediction of genotoxicity of 277 polycyclic aromatic compounds (PACs) directly from their molecular structures. Numerical descriptors encoding the topological, geometric, electronic, and polar surface area properties of the compounds were calculated to represent the structural information. Each compound's genotoxicity was represented with IMAX (maximal SOS induction factor) values measured by the SOS Chromotest in the presence and absence of S9 rat liver homogenate. The compounds' class identity was determined by a cutoff IMAX value of 1.25-compounds with IMAX > 1.25 in either test were classified as genotoxic, and the ones with IMAX < or = 1.25 were nongenotoxic. Several binary classification models were generated to predict genotoxicity: k-nearest neighbor (k-NN), linear discriminant analysis, and probabilistic neural network. The study showed k-NN to provide the highest predictive ability among the three classifiers with a training set classification rate of 93.5%. A consensus model was also developed that incorporated the three classifiers and correctly predicted 81.2% of the 277 compounds. It also provided a higher prediction rate on the genotoxic class than any other single model.

Animals↗