A new label fusion method using graph cuts: application to hippocampus segmentation

The aim of this paper is to develop a probabilistic modeling framework for the segmentation of structures of interest from a collection of atlases. Given a subset of registered atlases into the target image for a particular Region of Interest (ROI), a statistical model of appearance and shape is computed for fusing the labels. Segmentations are obtained by minimizing an energy function associated with the proposed model, using a graph-cut technique. We test different label fusion methods on publicly available MR images of human brains.

Desarrollo de metodologías para mejorar la estimación de los hidrogramas de diseño para el cálculo de los órganos de desagüe de las presas

En la presente Tesis se ha llevado a cabo el contraste y desarrollo de metodologías que permitan mejorar el cálculo de las avenidas de proyecto y extrema empleadas en el cálculo de la seguridad hidrológica de las presas. En primer lugar se ha abordado el tema del cálculo de las leyes de frecuencia de caudales máximos y su extrapolación a altos periodos de retorno. Esta cuestión es de gran relevancia, ya que la adopción de estándares de seguridad hidrológica para las presas cada vez más exigentes, implica la utilización de periodos de retorno de diseño muy elevados cuya estimación conlleva una gran incertidumbre. Es importante, en consecuencia incorporar al cálculo de los caudales de diseño todas la técnicas disponibles para reducir dicha incertidumbre. Asimismo, es importante hacer una buena selección del modelo estadístico (función de distribución y procedimiento de ajuste) de tal forma que se garantice tanto su capacidad para describir el comportamiento de la muestra, como para predecir de manera robusta los cuantiles de alto periodo de retorno. De esta forma, se han realizado estudios a escala nacional con el objetivo de determinar el esquema de regionalización que ofrece mejores resultados para las características hidrológicas de las cuencas españolas, respecto a los caudales máximos anuales, teniendo en cuenta el numero de datos disponibles. La metodología utilizada parte de la identificación de regiones homogéneas, cuyos límites se han determinado teniendo en cuenta las características fisiográficas y climáticas de las cuencas, y la variabilidad de sus estadísticos, comprobando posteriormente su homogeneidad. A continuación, se ha seleccionado el modelo estadístico de caudales máximos anuales con un mejor comportamiento en las distintas zonas de la España peninsular, tanto para describir los datos de la muestra como para extrapolar a los periodos de retorno más altos. El proceso de selección se ha basado, entre otras cosas, en la generación sintética de series de datos mediante simulaciones de Monte Carlo, y el análisis estadístico del conjunto de resultados obtenido a partir del ajuste de funciones de distribución a estas series bajo distintas hipótesis. Posteriormente, se ha abordado el tema de la relación caudal-volumen y la definición de los hidrogramas de diseño en base a la misma, cuestión que puede ser de gran importancia en el caso de presas con grandes volúmenes de embalse. Sin embargo, los procedimientos de cálculo hidrológico aplicados habitualmente no tienen en cuenta la dependencia estadística entre ambas variables. En esta Tesis se ha desarrollado un procedimiento para caracterizar dicha dependencia estadística de una manera sencilla y robusta, representando la función de distribución conjunta del caudal punta y el volumen en base a la función de distribución marginal del caudal punta y la función de distribución condicionada del volumen respecto al caudal. Esta última se determina mediante una función de distribución log-normal, aplicando un procedimiento de ajuste regional. Se propone su aplicación práctica a través de un procedimiento de cálculo probabilístico basado en la generación estocástica de un número elevado de hidrogramas. La aplicación a la seguridad hidrológica de las presas de este procedimiento requiere interpretar correctamente el concepto de periodo de retorno aplicado a variables hidrológicas bivariadas. Para ello, se realiza una propuesta de interpretación de dicho concepto. El periodo de retorno se entiende como el inverso de la probabilidad de superar un determinado nivel de embalse. Al relacionar este periodo de retorno con las variables hidrológicas, el hidrograma de diseño de la presa deja de ser un único hidrograma para convertirse en una familia de hidrogramas que generan un mismo nivel máximo en el embalse, representados mediante una curva en el plano caudal volumen. Esta familia de hidrogramas de diseño depende de la propia presa a diseñar, variando las curvas caudal-volumen en función, por ejemplo, del volumen de embalse o la longitud del aliviadero. El procedimiento propuesto se ilustra mediante su aplicación a dos casos de estudio. Finalmente, se ha abordado el tema del cálculo de las avenidas estacionales, cuestión fundamental a la hora de establecer la explotación de la presa, y que puede serlo también para estudiar la seguridad hidrológica de presas existentes. Sin embargo, el cálculo de estas avenidas es complejo y no está del todo claro hoy en día, y los procedimientos de cálculo habitualmente utilizados pueden presentar ciertos problemas. El cálculo en base al método estadístico de series parciales, o de máximos sobre un umbral, puede ser una alternativa válida que permite resolver esos problemas en aquellos casos en que la generación de las avenidas en las distintas estaciones se deba a un mismo tipo de evento. Se ha realizado un estudio con objeto de verificar si es adecuada en España la hipótesis de homogeneidad estadística de los datos de caudal de avenida correspondientes a distintas estaciones del año. Asimismo, se han analizado los periodos estacionales para los que es más apropiado realizar el estudio, cuestión de gran relevancia para garantizar que los resultados sean correctos, y se ha desarrollado un procedimiento sencillo para determinar el umbral de selección de los datos de tal manera que se garantice su independencia, una de las principales dificultades en la aplicación práctica de la técnica de las series parciales. Por otra parte, la aplicación practica de las leyes de frecuencia estacionales requiere interpretar correctamente el concepto de periodo de retorno para el caso estacional. Se propone un criterio para determinar los periodos de retorno estacionales de forma coherente con el periodo de retorno anual y con una distribución adecuada de la probabilidad entre las distintas estaciones. Por último, se expone un procedimiento para el cálculo de los caudales estacionales, ilustrándolo mediante su aplicación a un caso de estudio. The compare and develop of a methodology in order to improve the extreme flow estimation for dam hydrologic security has been developed. First, the work has been focused on the adjustment of maximum peak flows distribution functions from which to extrapolate values for high return periods. This has become a major issue as the adoption of stricter standards on dam hydrologic security involves estimation of high design return periods which entails great uncertainty. Accordingly, it is important to incorporate all available techniques for the estimation of design peak flows in order to reduce this uncertainty. Selection of the statistical model (distribution function and adjustment method) is also important since its ability to describe the sample and to make solid predictions for high return periods quantiles must be guaranteed. In order to provide practical application of previous methodologies, studies have been developed on a national scale with the aim of determining a regionalization scheme which features best results in terms of annual maximum peak flows for hydrologic characteristics of Spanish basins taking into account the length of available data. Applied methodology starts with the delimitation of regions taking into account basin’s physiographic and climatic characteristics and the variability of their statistical properties, and continues with their homogeneity testing. Then, a statistical model for maximum annual peak flows is selected with the best behaviour for the different regions in peninsular Spain in terms of describing sample data and making solid predictions for high return periods. This selection has been based, among others, on synthetic data series generation using Monte Carlo simulations and statistical analysis of results from distribution functions adjustment following different hypothesis. Secondly, the work has been focused on the analysis of the relationship between peak flow and volume and how to define design flood hydrographs based on this relationship which can be highly important for large volume reservoirs. However, commonly used hydrologic procedures do not take statistical dependence between these variables into account. A simple and sound method for statistical dependence characterization has been developed by the representation of a joint distribution function of maximum peak flow and volume which is based on marginal distribution function of peak flow and conditional distribution function of volume for a given peak flow. The last one is determined by a regional adjustment procedure of a log-normal distribution function. Practical application is proposed by a probabilistic estimation procedure based on stochastic generation of a large number of hydrographs. The use of this procedure for dam hydrologic security requires a proper interpretation of the return period concept applied to bivariate hydrologic data. A standard is proposed in which it is understood as the inverse of the probability of exceeding a determined reservoir level. When relating return period and hydrological variables the only design flood hydrograph changes into a family of hydrographs which generate the same maximum reservoir level and that are represented by a curve in the peak flow-volume two-dimensional space. This family of design flood hydrographs depends on the dam characteristics as for example reservoir volume or spillway length. Two study cases illustrate the application of the developed methodology. Finally, the work has been focused on the calculation of seasonal floods which are essential when determining the reservoir operation and which can be also fundamental in terms of analysing the hydrologic security of existing reservoirs. However, seasonal flood calculation is complex and nowadays it is not totally clear. Calculation procedures commonly used may present certain problems. Statistical partial duration series, or peaks over threshold method, can be an alternative approach for their calculation that allow to solve problems encountered when the same type of event is responsible of floods in different seasons. A study has been developed to verify the hypothesis of statistical homogeneity of peak flows for different seasons in Spain. Appropriate seasonal periods have been analyzed which is highly relevant to guarantee correct results. In addition, a simple procedure has been defined to determine data selection threshold on a way that ensures its independency which is one of the main difficulties in practical application of partial series. Moreover, practical application of seasonal frequency laws requires a correct interpretation of the concept of seasonal return period. A standard is proposed in order to determine seasonal return periods coherently with the annual return period and with an adequate seasonal probability distribution. Finally a methodology is proposed to calculate seasonal peak flows. A study case illustrates the application of the proposed methodology.

From patterns to processes: Phase and density dependencies in the Canadian lynx cycle

Across the boreal forest of North America, lynx populations undergo 10-year cycles. Analysis of 21 time series from 1821 to the present demonstrates that these fluctuations are generated by nonlinear processes with regulatory delays. Trophic interactions between lynx and hares cause delayed density-dependent regulation of lynx population growth. The nonlinearity, in contrast, appears to arise from phase dependencies in hunting success by lynx through the cycle. Using a combined approach of empirical, statistical, and mathematical modeling, we highlight how shifts in trophic interactions between the lynx and the hare generate the nonlinear process primarily by shifting functional response curves during the increase and the decrease phases.

Cortisol fecal em ovinos: curva de excreção e estabilidade

O presente estudo foi dividido em dois experimentos, tendo como objetivo determinar a curva de excreção do cortisol fecal e sua estabilidade nas fezes perante exposição à diferentes períodos de tempo e temperatura entre as colheitas e análises, correlacionando os níveis de cortisol fecal com o pico de cortisol sanguíneo. No experimento 1, seis fêmeas mestiças (Dorper x Santa Inês) tiveram suas fezes totais colhidas durante 24 horas após a aplicação do hormônio adrenocorticotrófico (ACTH), além de colheitas de sangue realizadas antes da aplicação do ACTH, 60, 120 e 300 minutos depois; durante as quais foram atribuídos escores de reatividade para cada animal. Logo após as análises foi iniciado o experimento 2, no qual 9 cordeiros mestiços (Dorper x Santa Inês) foram submetidos a uma situação de estresse térmico durante os horários das 11 às 15 horas da tarde, tendo suas fezes colhidas às 23 horas do mesmo dia. Após a colheita, as fezes foram agrupadas e homogeneizadas em três grupos distintos, de onde retiraram-se alíquotas referentes aos tratamentos propostos: três temperaturas (15°, 25° e 35°) e quatro tempos (1, 3, 6 e 12 horas). Os dados da curva de excreção foram analisados por ANOVA, bem como pela correlação entre os valores de cortisol sanguíneo, fecal e reatividade. Para análise da estabilidade foi utilizada ANOVA multifatorial com dois fatores (temperatura e intervalo de tempo). Para avaliação das variáveis comportamentais foi realizada a transformação de escala dos dados para \"arco-seno raiz de porcentagem\", procedendo-se à análise de variância. O modelo estatístico contemplou os efeitos de dia (1, 2 e 3) com análise individual por animal. Os parâmetros de cortisol sanguíneo, frequência respiratória e temperatura retal foram analisados pelo teste t e correlação de Pearson. Todas as comparações de médias foram realizadas por teste F e teste t (PDIFF). A reatividade durante a colheita não exerceu efeito significativo sobre os valores de cortisol sanguíneo, os quais demonstraram médias maiores 60 minutos após a aplicação do ACTH e, após 300 minutos as ovelhas apresentaram níveis de cortisol considerados normais para ovinos sem estresse. Por outro lado, o pico de cortisol nas fezes foi verificado aproximadamente 10 a 12 horas após o pico de cortisol no sangue, não sendo verificadas diminuições significativas nas concentrações que indicassem o retorno aos níveis basais durante o período de 24 horas (P>0,05). Não foram observadas diferenças significativas entre os tempos e temperaturas aos quais as amostras de fezes foram submetidas (P>0,05), verificando-se uma tendência a manutenção da concentração do cortisol fecal em ovinos durante o período de 12 horas após a colheita.

Homogeneous Spatial Units (HSU) - a Pan-European geographical basis for environmental and socio-economic modelling

The spatial data set delineates areas with similar environmental properties regarding soil, terrain morphology, climate and affiliation to the same administrative unit (NUTS3 or comparable units in size) at a minimum pixel size of 1km2. The scope of developing this data set is to provide a link between spatial environmental information (e.g. soil properties) and statistical data (e.g. crop distribution) available at administrative level. Impact assessment of agricultural management on emissions of pollutants or radiative active gases, or analysis regarding the influence of agricultural management on the supply of ecosystem services, require the proper spatial coincidence of the driving factors. The HSU data set provides e.g. the link between the agro-economic model CAPRI and biophysical assessment of environmental impacts (updating previously spatial units, Leip et al. 2008), for the analysis of policy scenarios. Recently, a statistical model to disaggregate crop information available from regional statistics to the HSU has been developed (Lamboni et al. 2016). The HSU data set consists of the spatial layers provided in vector and raster format as well as attribute tables with information on the properties of the HSU. All input data for the delineation the HSU is publicly available. For some parameters the attribute tables provide the link between the HSU data set and e.g. the soil map(s) rather than the data itself. The HSU data set is closely linked the USCIE data set.

The North Sea autumn spawning Herring (Clupea harengus L.) Spawning Component Abundance Index (SCAI)

The North Sea autumn-spawning herring (Clupea harengus) stock consists of a set of different spawning components. The dynamics of the entire stock have been well characterized, but although time-series of larval abundance indices are available for the individual components, study of the dynamics at the component level has historically been hampered by missing observations and high sampling noise. A simple state-space statistical model is developed that is robust to these problems, gives a good fit to the data, and proves capable of both handling and predicting missing observations well. Furthermore, the sum of the fitted abundance indices across all components proves an excellent proxy for the biomass of the total stock, even though the model utilizes information at the individual-component level. The Orkney-Shetland component appears to have recovered faster from historic depletion events than the other components, whereas the Downs component has been the slowest. These differences give rise to changes in stock composition, which are shown to vary widely within a relatively short time. The modelling framework provides a valuable tool for studying and monitoring the dynamics of the individual components of the North Sea herring stock.

Social and behavioural determinants of early childhood caries

Background: To investigate the association between selected social and behavioural (infant feeding and preventive dental practices) variables and the presence of early childhood caries in preschool children within the north Brisbane region. Methods: A cross sectional sample of 2515 children aged four to five years were examined in a preschool setting using prevalence (percentage with caries) and severity (dmft) indices. A self-administered questionnaire obtained information regarding selected social and behavioural variables. The data were modelled using multiple logistic regression analysis at the 5 per cent level of significance. Results: The final explanatory model for caries presence in four to five year old children included the variables breast feeding from three to six months of age (OR=0.7, CI=0.5, 1.0), sleeping with the bottle (OR=1.9, CI=1.5, 2.4), sipping from the bottle (OR=1.6, CI=1.2, 2.0), ethnicity other than Caucasian (OR=1.9, CI=1.4, 2.5), annual family income $20,000-$35,000 (OR = 1.7, CI=1.3, 2.3) and annual family income less than $20,000 (OR=2.1, CI=1.5, 2.8). Conclusion: A statistical model for early childhood caries in preschool children within the north Brisbane region has been constructed using selected social and behavioural determinants. Epidemiological data can be used for improved public oral health service planning and resource allocation within the region.

A vulnerability analysis of the temperate forests of south central Chile A vulnerability analysis of the temperate forests of south central Chile. Biological Conservation, 122 2005: 9-21.

Areas of the landscape that are priorities for conservation should be those that are both vulnerable to threatening processes and that if lost or degraded, will result in conservation targets being compromised. While much attention is directed towards understanding the patterns of biodiversity, much less is given to determining the areas of the landscape most vulnerable to threats. We assessed the relative vulnerability of remaining areas of native forest to conversion to plantations in the ecologically significant temperate rainforest region of south central Chile. The area of the study region is 4.2 million ha and the extent of plantations is approximately 200000 ha. First, the spatial distribution of native forest conversion to plantations was determined. The variables related to the spatial distribution of this threatening process were identified through the development of a classification tree and the generation of a multivariate. spatially explicit, statistical model. The model of native forest conversion explained 43% of the deviance and the discrimination ability of the model was high. Predictions were made of where native forest conversion is likely to occur in the future. Due to patterns of climate, topography, soils and proximity to infrastructure and towns, remaining forest areas differ in their relative risk of being converted to plantations. Another factor that may increase the vulnerability of remaining native forest in a subset of the study region is the proposed construction of a highway. We found that 90% of the area of existing plantations within this region is within 2.5 km of roads. When the predictions of native forest conversion were recalculated accounting for the construction of this highway, it was found that: approximately 27000 ha of native forest had an increased probability of conversion. The areas of native forest identified to be vulnerable to conversion are outside of the existing reserve network. (C) 2004 Elsevier Ltd. All tights reserved.

Detecting finite bandwidth periodic signals in stationary noise using the signal coherence spectrum

All signals that appear to be periodic have some sort of variability from period to period regardless of how stable they appear to be in a data plot. A true sinusoidal time series is a deterministic function of time that never changes and thus has zero bandwidth around the sinusoid's frequency. A zero bandwidth is impossible in nature since all signals have some intrinsic variability over time. Deterministic sinusoids are used to model cycles as a mathematical convenience. Hinich [IEEE J. Oceanic Eng. 25 (2) (2000) 256-261] introduced a parametric statistical model, called the randomly modulated periodicity (RMP) that allows one to capture the intrinsic variability of a cycle. As with a deterministic periodic signal the RMP can have a number of harmonics. The likelihood ratio test for this model when the amplitudes and phases are known is given in [M.J. Hinich, Signal Processing 83 (2003) 1349-13521. A method for detecting a RMP whose amplitudes and phases are unknown random process plus a stationary noise process is addressed in this paper. The only assumption on the additive noise is that it has finite dependence and finite moments. Using simulations based on a simple RMP model we show a case where the new method can detect the signal when the signal is not detectable in a standard waterfall spectrograrn display. (c) 2005 Elsevier B.V. All rights reserved.

Predicting functional capacity outcome 12 months after hospitalized injury

Background: There is a recognized need to move from mortality to morbidity outcome predictions following traumatic injury. However, there are few morbidity outcome prediction scoring methods and these fail to incorporate important comorbidities or cofactors. This study aims to develop and evaluate a method that includes such variables. Methods: This was a consecutive case series registered in the Queensland Trauma Registry that consented to a prospective 12-month telephone conducted follow-up study. A multivariable statistical model was developed relating Trauma Registry data to trichotomized 12-month post-injury outcome (categories: no limitations, minor limitations and major limitations). Cross-validation techniques using successive single hold-out samples were then conducted to evaluate the model's predictive capabilities. Results: In total, 619 participated, with 337 (54%) experiencing no limitations, 101 (16%) experiencing minor limitations and 181 (29%) experiencing major limitations 12 months after injury. The final parsimonious multivariable statistical model included whether the injury was in the lower extremity body region, injury severity, age, length of hospital stay, pulse at admission and whether the participant was admitted to an intensive care unit. This model explained 21% of the variability in post-injury outcome. Predictively, 64% of those with no limitations, 18% of those with minor limitations and 37% of those with major limitations were correctly identified. Conclusion: Although carefully developed, this statistical model lacks the predictive power necessary for its use as a basis of a useful prognostic tool. Further research is required to identify variables other than those routinely used in the Trauma Registry to develop a model with the necessary predictive utility.

The inhibitory effects of free nitrous acid on the energy generation and growth processes of an enriched Nitrobacter culture

The inhibitory effects of nitrite (NO2-)/free nitrous acid (HNO2-FNA) on the metabolism of Nitrobacter were investigated using a method allowing the decoupling of the growth and energy generation processes. A lab-scale sequencing batch reactor was operated for the enrichment of a Nitrobacter culture. Fluorescent in situ hybridization (FISH) analysis showed that 73% of the bacterial population was Nitrobacter. Batch tests were carried out to assess the oxygen and nitrite consumption rates of the enriched culture at low and high nitrite levels, in the presence or absence of inorganic carbon. It was observed that in the absence of CO2, the Nitrobacter culture was able to oxidize nitrite at a rate that is 76% of that in the presence of CO2, with an oxygen consumption rate that is 85% of that measured in the presence of CO2. This enabled the impacts of nitrite/FNA on the catabolic and anabolic processes of Nitrobacter to be assessed separately. FNA rather than nitrite was likely the actual inhibitor to the Nitrobacter metabolism. It was revealed that FNA of up to 0.05 mg HNO2-N center dot L-1 (3.4 mu M), which was the highest FNA concentration used in this study, did not have any inhibitory effect on the catabolic processes of Nitrobacter. However, FNA initiated its inhibition to the anabolic processes of Nitrobacter at approximately 0.011 mg HNO2-N center dot L-1 (0.8 mu M), and completely stopped biomass synthesis at a concentration of approximately 0.023 mg HNO2-N center dot L-1 (1.6 mu M). The inhibitory effect could be described by an empirical inhibitory model proposed in this paper, but the underlying mechanisms remain to be revealed.

EXISTE DIFERENÇA ENTRE OS RETORNOS DAS EMPRESAS LISTADAS NO IBOVESPA E NOS NÍVEIS DE GOVERNANÇA CORPORATIVA?

Esta pesquisa teve como objetivo verificar se existe diferença entre os retornos das Empresas listadas no IBOVESPA e nos Níveis de Governança Corporativa criados pela BOVESPA em dezembro de 2000, visando diferenciar as empresas que voluntariamente adotassem práticas adicionais de governança corporativa. O principal objetivo desde processo é melhorar a transparência entre o investidor e as empresas, reduzindo assim a assimetria de informação. Para efetuar esta pesquisa primeiramente foi realizada uma revisão bibliográfica sobre a assimetria da informação, teoria dos custos de transação, teoria da agência e um histórico da governança corporativa no mundo e no Brasil. Para verificar se existe diferença entre os retornos das empresas listadas no IBOVESPA e nos níveis diferenciados de governança corporativa utilizou-se o modelo estatístico ANOVA e Teste t em uma amostra composta por todas as empresas listadas no IBOVESPA em 31/03/2011 que tiveram ações negociadas entre 2006 e 2010 que são os últimos cinco anos da implantação dos níveis de governança no Brasil. Como resultado obteve-se que as empresa listadas no Novo Mercado e no Nível 1 possuem maior retorno que as empresas listadas no mercado tradicional.

The influence of high contrast acuity and normalised low contrast acuity upon self-reported situation avoidance and driving crashes

The aim of this study was to determine the cues used to signal avoidance of difficult driving situations and to test the hypothesis that drivers with relatively poor high contrast visual acuity (HCVA) have fewer crashes than drivers with relatively poor normalised low contrast visual acuity (NLCVA). This is because those with poorer HCVA are well aware of their difficulties and avoid dangerous driving situations while those poorer NLCVA are often unaware of the extent of their problem. Age, self-reported situation avoidance and HCVA were collected during a practice based study of 690 drivers. Screening was also carried out on 7254 drivers at various venues, mainly motorway sites, throughout the UK. Age, self-reported situation avoidance and prior crash involvement were recorded and Titmus vision screeners were used to measure HCVA and NLCVA. Situation avoidance increased in reduced visibility conditions and was influenced by age and HCVA. Only half of the drivers used visual cues to signal situation avoidance and most of these drivers used high rather than low contrast cues. A statistical model designed to remove confounding interrelationships between variables showed, for drivers that did not report situation avoidance, that crash involvement decreased for drivers with below average HCVA and increased for those with below average NLCVA. These relationships accounted for less than 1% of the crash variance, so the hypothesis was not strongly supported. © 2002 The College of Optometrists.

The application of analysis of variance (ANOVA) to different experimental designs in optometry

Analysis of variance (ANOVA) is the most efficient method available for the analysis of experimental data. Analysis of variance is a method of considerable complexity and subtlety, with many different variations, each of which applies in a particular experimental context. Hence, it is possible to apply the wrong type of ANOVA to data and, therefore, to draw an erroneous conclusion from an experiment. This article reviews the types of ANOVA most likely to arise in clinical experiments in optometry including the one-way ANOVA ('fixed' and 'random effect' models), two-way ANOVA in randomised blocks, three-way ANOVA, and factorial experimental designs (including the varieties known as 'split-plot' and 'repeated measures'). For each ANOVA, the appropriate experimental design is described, a statistical model is formulated, and the advantages and limitations of each type of design discussed. In addition, the problems of non-conformity to the statistical model and determination of the number of replications are considered. © 2002 The College of Optometrists.

Detection of signals corrupted by the combination of Gaussian and Non-Gaussian noise

The detection of signals in the presence of noise is one of the most basic and important problems encountered by communication engineers. Although the literature abounds with analyses of communications in Gaussian noise, relatively little work has appeared dealing with communications in non-Gaussian noise. In this thesis several digital communication systems disturbed by non-Gaussian noise are analysed. The thesis is divided into two main parts. In the first part, a filtered-Poisson impulse noise model is utilized to calulate error probability characteristics of a linear receiver operating in additive impulsive noise. Firstly the effect that non-Gaussian interference has on the performance of a receiver that has been optimized for Gaussian noise is determined. The factors affecting the choice of modulation scheme so as to minimize the deterimental effects of non-Gaussian noise are then discussed. In the second part, a new theoretical model of impulsive noise that fits well with the observed statistics of noise in radio channels below 100 MHz has been developed. This empirical noise model is applied to the detection of known signals in the presence of noise to determine the optimal receiver structure. The performance of such a detector has been assessed and is found to depend on the signal shape, the time-bandwidth product, as well as the signal-to-noise ratio. The optimal signal to minimize the probability of error of; the detector is determined. Attention is then turned to the problem of threshold detection. Detector structure, large sample performance and robustness against errors in the detector parameters are examined. Finally, estimators of such parameters as. the occurrence of an impulse and the parameters in an empirical noise model are developed for the case of an adaptive system with slowly varying conditions.