On Convergence Properties of the EM Algorithm for Gaussian Mixtures

"Expectation-Maximization'' (EM) algorithm and gradient-based approaches for maximum likelihood learning of finite Gaussian mixtures. We show that the EM step in parameter space is obtained from the gradient via a projection matrix $P$, and we provide an explicit expression for the matrix. We then analyze the convergence of EM in terms of special properties of $P$ and provide new results analyzing the effect that $P$ has on the likelihood surface. Based on these mathematical results, we present a comparative discussion of the advantages and disadvantages of EM and other algorithms for the learning of Gaussian mixture models.

Hierarchical Mixtures of Experts and the EM Algorithm

We present a tree-structured architecture for supervised learning. The statistical model underlying the architecture is a hierarchical mixture model in which both the mixture coefficients and the mixture components are generalized linear models (GLIM's). Learning is treated as a maximum likelihood problem; in particular, we present an Expectation-Maximization (EM) algorithm for adjusting the parameters of the architecture. We also develop an on-line learning algorithm in which the parameters are updated incrementally. Comparative simulation results are presented in the robot dynamics domain.

Some theoretical and methodological considerations for the development of a critical competences model (CCM): an operant approach

This paper develops some theoretical and methodological considerations for the development of a critical competence model (CCM). The model is defined as a set of skills and knowledge functionally organized allowing measurable results with positive consequences for the strategic business objectives. The theoretical approaches of classical model of competences, the contemporary model of competencies and human competencies model were revised for the proposal development. implementation of the model includes 5 steps: 1) conducting a job analysis considering which dimensions or facets are subject to revision, 2) identify people with the opposite performance (the higher performance and lower performance); 3) identify critical incidents most relevant to the job position, 4) develop behavioral expectation scales (bes) and 5) validate BES obtained for experts in the field. As a final consideration, is determined that the competence models require accurate measurement. Approaches considering excessive theoreticism may cause the issue of competence become a fashion business with low or minimal impact, affecting its validity, reliability and deployment in organizations.

The Peruvian Citizen Perception and Expectation toward the e-Government. The Electronic Tax Payment Successful e-Gov Project

En este capítulo, los autores tratan de demostrar como los gobierno latinoamericanos, específicamente el peruano, evalúan los esfuerzos que realizan en la implementación de iniciativas de gobierno electrónico, identificando las expectativas que tienen los ciudadanos frente a estas iniciativas y como las perciben, aplicando un modelo de evaluación al servicio de  pago electrónico de impuestos implementado como una iniciativa exitosa en Latinoamérica. El modelo propuesto fue desarrollado por uno de los autores como parte de su tesis doctoral y evaluado por el otro como jurado en el tribunal de la misma. Incluye cinco constructos latentes independientes, actitud, aptitud, confianza, relevancia y satisfacción que afecta a dos constructos latentes dependientes, la percepción y la expectativa frente a su relación con la administración pública vía el uso de las tecnologías de la información y las comunicaciones – TIC, bajo el efecto de variables ilustrativas relacionadas con los ciudadanos, las instituciones y el contexto.

The timing of Quaternary calcrete development in semi-arid southeast Spain: Investigating the role of climate on calcrete genesis

Quaternary-aged calcrete horizons are common weathering products in arid and semi-arid regions. It is, however, unclear how calcrete forming processes respond to the major oscillations in climate that occur over the Quaternary period. This paper presents a U-series-based calcrete age database from the Sorbas basin, southeast Spain. The study constructs an age frequency distribution of these ages which is consequently compared to a range of palaeoenvironmental records from the Mediterranean. The age distribution presented here suggests that the formation of pedogenic calcrete horizons in the Sorbas basin primarily occurs during 'warm' isotope stages (MIS 1 and 5), with very few calcrete ages occurring during cold glacial/stadial stages (MIS 2, 3 and 4). It is suggested that this is a function of the environments that existed during 'warm' isotope stages being more conducive to calcrete development than those that existed during cold climate episodes. In a semi-arid region such as the Sorbas basin it is likely that increased aridity during glacial stages, coupled with reduced vegetation and accelerated landscape instability, was crucial in reducing rates of calcrete formation. In a semi-arid region such as southeast Spain, calcrete formation during the Quaternary, therefore, oscillates with climate change but is primarily a "warm" episode phenomenon. It is suggested that further studies are required to see how calcrete genesis responds to environmental change in more humid parts of the Mediterranean. (C) 2009 Elsevier B.V. All rights reserved.

U-series isochron dating of immature and mature calcretes as a basis for constructing Quaternary landform chronologies for the Sorbas basin, southeast Spain

Immature and mature calcretes from an alluvial terrace sequence in the Sorbas basin, southeast Spain, were dated by the U-series isochron technique. The immature horizons consistently produced statistically reliable ages of high precision. The mature horizons typically produced statistically unreliable ages but, because of linear trends in the dataset and low errors associated with each data point, it was still possible to place a best-fit isochron through the dataset to produce an age with low associated uncertainties. It is, however, only possible to prove that these statistically unreliable ages have geochronological significance if multiple isochron ages are produced for a single site, and if these multiple ages are stratigraphically consistent. The geochronological significance of such ages can be further proven if at least one of the multiple ages is statistically reliable. By using this technique to date calcretes that have formed during terrace aggradation and at the terrace surface after terrace abandonment it is possible not only to date the timing of terrace aggradation but also to constrain the age at which the river switched from aggradation to incision. This approach, therefore, constrains the timing of changes in fluvial processes more reliably than any currently used geochronological procedure and is appropriate for dating terrace sequences in dryland regions worldwide, wherever calcrete horizons are present. (c) 2005 University of Washington. All rights reserved.

Mineral and geochemical characterization of a leptic aluandic soil and a thapto aluandic-ferralsol developed on trachytes in Mount Bambouto (Cameroon volcanic line)

Mineral and geochemical investigations were carried out on soil samples and fresh rock (trachytes) from two selected soil profiles (TM profile on leptic aluandic soils and TL profile on thapto aluandic-ferralsols) from Mount Bambouto to better understand geochemical processes and mineral paragenesis involved in the development of soils in this environment. In TM profile, the hydrated halloysites and goethite occur in the weathered saprolite boulders of BC horizon while dehydrated halloysite, gibbsite and goethite dominate the soils matrices of BC and A horizons. In TL profile, the dehydrated halloysites and goethite are the most abundant secondary minerals in the weathered saprolites of C and BC horizons while gibbsite, hematite and kaolinite occur in the soil matrices of BC, B and A horizons. The highest gibbsite content is in the platy nodules of B horizon. In both soil profiles, organo-metal complexes (most likely of AI and Fe) are present in the surface A horizon. Geochemically, between the fresh rock and the weathered saprolites in both soils, SiO2, K2O, CaO, Na2O and MgO contents decrease strongly while Fe2O3 and Al2O3 tend to accumulate. The molar ratio of SiO2/Al2O3 (Ki) and the sum of Ca, Mg, K and Na ions (TRB) also decreases abruptly between fresh rocks and the weathered saprolites, but increases significantly at the soil surface. The TM profile shows intense Al enrichment whereas the TL profile highlights enrichment in both AI and Fe as the weathering progresses upwards. Both soil profiles are enriched in Ni, Cu, Ba and Co and depleted in U, Th, Ta, Hf, Y, Sr, Pb, Zr and Zn relative to fresh rock. They also show a relatively low fractionation of the rare earth elements (REE: La, Nd, Sm, Eu, Tb, Yb and Lu), except for Ce which tends to be enriched in soils compared to CI chondrite. All these results give evidence of intense hydrolysis at soil deep in Mount Bambouto resulting in the formation of halloysite which progressively transforms into gibbsite and/or dehydrated halloysite. At the soil surface, the prominent pedogenetic process refers to andosolization with formation of organo-metal complexes. In TL profile, the presence of kaolinite in soil matrices BC and B horizons is consistent with ferralitization at soil deep. In conclusion, soil forming processes in Mount Bambouto are strongly influenced by local climate: (i) in the upper mountain (>2000 m), the fresh, misty and humid climate favors andosolization; whereas (ii) in the middle lands (1700-2000 m) with a relatively dry climate, both andosolization at the soil surface and ferralitization at soil deep act together. (C) 2009 Elsevier B.V. All rights reserved.

How soccer players head the ball: a test of optic acceleration cancellation theory with virtual reality

We measured the movements of soccer players heading a football in a fully immersive virtual reality environment. In mid-flight the ball’s trajectory was altered from its normal quasi-parabolic path to a linear one, producing a jump in the rate of change of the angle of elevation of gaze (α) from player to ball. One reaction time later the players adjusted their speed so that the rate of change of α increased when it had been reduced and reduced it when it had been increased. Since the result of the player’s movement was to regain a value of the rate of change close to that before the disturbance, the data suggest that the players have an expectation of, and memory for, the pattern that the rate of change of α will follow during the flight. The results support the general claim that players intercepting balls use servo control strategies and are consistent with the particular claim of Optic Acceleration Cancellation theory that the servo strategy is to allow α to increase at a steadily decreasing rate.

Use of a crop model ensemble to quantify CO2 stimulation of water-stressed and well-watered crops

Increased atmospheric concentrations of carbon dioxide (CO2) will benefit the yield of most crops. Two free air CO2 enrichment (FACE) meta-analyses have shown increases in yield of between 0 and 73% for C3 crops. Despite this large range, few crop modelling studies quantify the uncertainty inherent in the parameterisation of crop growth and development. We present a novel perturbed-parameter method of crop model simulation, which uses some constraints from observations, that does this. The model used is the groundnut (i.e. peanut; Arachis hypogaea L.) version of the general large-area model for annual crops (GLAM). The conclusions are of relevance to C3 crops in general. The increases in yield simulated by GLAM for doubled CO2 were between 16 and 62%. The difference in mean percentage increase between well-watered and water-stressed simulations was 6.8. These results were compared to FACE and controlled environment studies, and to sensitivity tests on two other crop models of differing levels of complexity: CROPGRO, and the groundnut model of Hammer et al. [Hammer, G.L., Sinclair, T.R., Boote, K.J., Wright, G.C., Meinke, H., Bell, M.J., 1995. A peanut simulation model. I. Model development and testing. Agron. J. 87, 1085-1093]. The relationship between CO2 and water stress in the experiments and in the models was examined. From a physiological perspective, water-stressed crops are expected to show greater CO2 stimulation than well-watered crops. This expectation has been cited in literature. However, this result is not seen consistently in either the FACE studies or in the crop models. In contrast, leaf-level models of assimilation do consistently show this result. An analysis of the evidence from these models and from the data suggests that scale (canopy versus leaf), model calibration, and model complexity are factors in determining the sign and magnitude of the interaction between CO2 and water stress. We conclude from our study that the statement that 'water-stressed crops show greater CO2 stimulation than well-watered crops' cannot be held to be universally true. We also conclude, preliminarily, that the relationship between water stress and assimilation varies with scale. Accordingly, we provide some suggestions on how studies of a similar nature, using crop models of a range of complexity, could contribute further to understanding the roles of model calibration, model complexity and scale. (C) 2008 Elsevier B.V. All rights reserved.

Typification of farming systems for constructing representative farm models: two illustrations of the application of multi-variate analyses in Chile and Pakistan

If the fundamental precepts of Farming Systems Research were to be taken literally then it would imply that for each farm 'unique' solutions should be sought. This is an unrealistic expectation, but it has led to the idea of a recommendation domain, implying creating a taxonomy of farms, in order to increase the general applicability of recommendations. Mathematical programming models are an established means of generating recommended solutions, but for such models to be effective they have to be constructed for 'truly' typical or representative situations. The multi-variate statistical techniques provide a means of creating the required typologies, particularly when an exhaustive database is available. This paper illustrates the application of this methodology in two different studies that shared the common purpose of identifying types of farming systems in their respective study areas. The issues related with the use of factor and cluster analyses for farm typification prior to building representative mathematical programming models for Chile and Pakistan are highlighted. (C) 2003 Elsevier Science Ltd. All rights reserved.

Functional benefits of predator species diversity depend on prey identity

1. Determining the functional significance of species diversity in natural enemy assemblages is a key step towards prediction of the likely impact of biodiversity loss on natural pest control processes. While the biological control literature contains examples in which increased natural enemy diversity hinders pest control, other studies have highlighted mechanisms where pest suppression is promoted by increased enemy diversity. 2. This study aimed to test whether increased predator species diversity results in higher rates of predation on two key, but contrasting, insect pest species commonly found in the rice ecosystems of south-east Asia. 3. Glasshouse experiments were undertaken in which four life stages of a planthopper (Nilaparvata lugens) and a moth (Marasmia patnalis) were caged with single or three-species combinations of generalist predators. 4. Generally, predation rates of the three-species assemblages exceeded expectation when attacking M. patnalis, but not when attacking N. lugens. In addition, a positive effect of increased predator species richness on overall predation rate was found with M. patnalis but not with N. lugens. 5. The results are consistent with theoretical predictions that morphological and behavioural differentiation among prey life stages promotes functional complementarity among predator species. This indicates that emergent species diversity effects in natural enemy assemblages are context dependent; they depend not only on the characteristics of the predators species, but on the identity of the species on which they prey.

How a sequential design would have affected the GAIN international study of Gavestinel in stroke

While planning the GAIN International Study of gavestinel in acute stroke, a sequential triangular test was proposed but not implemented. Before the trial commenced it was agreed to evaluate the sequential design retrospectively to evaluate the differences in the resulting analyses, trial durations and sample sizes in order to assess the potential of sequential procedures for future stroke trials. This paper presents four sequential reconstructions of the GAIN study made under various scenarios. For the data as observed, the sequential design would have reduced the trial sample size by 234 patients and shortened its duration by 3 or 4 months. Had the study not achieved a recruitment rate that far exceeded expectation, the advantages of the sequential design would have been much greater. Sequential designs appear to be an attractive option for trials in stroke. Copyright 2004 S. Karger AG, Basel

Comparative micromorphology of nectariferous and nectarless labellar spurs in selected clades of subtribe Orchidinae (Orchidaceae)

Floral nectar spurs are widely considered to influence pollinator behaviour in orchids. Spurs of 21 orchid species selected from within four molecularly circumscribed clades of subtribe Orchidinae (based on Platanthera s.l., Gymnadenia-Dactylorhiza s.l., Anacamptis s.l., Orchis s.s.) were examined under light and scanning electron microscopes in order to estimate correlations between nectar production (categorized as absent, trace, reservoir), interior epidermal papillae (categorized as absent, short, medium, long) and epidermal cell striations (categorized as apparently absent, weak, moderate, strong). Closely related congeneric species scored similarly, but more divergent species showed less evidence of phylogenetic constraints. Nectar secretion was negatively correlated with striations and positively correlated with papillae, which were especially frequent and large in species producing substantial reservoirs of nectar. We speculate that the primary function of the papillae is conserving energy through nectar resorption and explain the presence of large papillae in a minority of deceit-pollinated species by arguing that the papillae improve pollination because they are a tactile expectation of pollinating insects. In contrast, the prominence of striations may be a 'spandrel', simply reflecting the thickness of the overlying cuticle. Developmentally, the spur is an invagination of the labellum; it is primarily vascularized by a single 'U'-shaped primary strand, with smaller strands present in some species. Several suggestions are made for developing further, more targeted research programmes. (C) 2009 The Linnean Society of London, Botanical Journal of the Linnean Society, 2009, 160, 369-387.