Modelli non lineari statici e dinamici su dati microeconomici: analisi delle condizioni finanziarie delle famiglie italiane

The thesis studies the economic and financial conditions of Italian households, by using microeconomic data of the Survey on Household Income and Wealth (SHIW) over the period 1998-2006. It develops along two lines of enquiry. First it studies the determinants of households holdings of assets and liabilities and estimates their correlation degree. After a review of the literature, it estimates two non-linear multivariate models on the interactions between assets and liabilities with repeated cross-sections. Second, it analyses households financial difficulties. It defines a quantitative measure of financial distress and tests, by means of non-linear dynamic probit models, whether the probability of experiencing financial difficulties is persistent over time. Chapter 1 provides a critical review of the theoretical and empirical literature on the estimation of assets and liabilities holdings, on their interactions and on households net wealth. The review stresses the fact that a large part of the literature explain households debt holdings as a function, among others, of net wealth, an assumption that runs into possible endogeneity problems. Chapter 2 defines two non-linear multivariate models to study the interactions between assets and liabilities held by Italian households. Estimation refers to a pooling of cross-sections of SHIW. The first model is a bivariate tobit that estimates factors affecting assets and liabilities and their degree of correlation with results coherent with theoretical expectations. To tackle the presence of non normality and heteroskedasticity in the error term, generating non consistent tobit estimators, semi-parametric estimates are provided that confirm the results of the tobit model. The second model is a quadrivariate probit on three different assets (safe, risky and real) and total liabilities; the results show the expected patterns of interdependence suggested by theoretical considerations. Chapter 3 reviews the methodologies for estimating non-linear dynamic panel data models, drawing attention to the problems to be dealt with to obtain consistent estimators. Specific attention is given to the initial condition problem raised by the inclusion of the lagged dependent variable in the set of explanatory variables. The advantage of using dynamic panel data models lies in the fact that they allow to simultaneously account for true state dependence, via the lagged variable, and unobserved heterogeneity via individual effects specification. Chapter 4 applies the models reviewed in Chapter 3 to analyse financial difficulties of Italian households, by using information on net wealth as provided in the panel component of the SHIW. The aim is to test whether households persistently experience financial difficulties over time. A thorough discussion is provided of the alternative approaches proposed by the literature (subjective/qualitative indicators versus quantitative indexes) to identify households in financial distress. Households in financial difficulties are identified as those holding amounts of net wealth lower than the value corresponding to the first quartile of net wealth distribution. Estimation is conducted via four different methods: the pooled probit model, the random effects probit model with exogenous initial conditions, the Heckman model and the recently developed Wooldridge model. Results obtained from all estimators accept the null hypothesis of true state dependence and show that, according with the literature, less sophisticated models, namely the pooled and exogenous models, over-estimate such persistence.

Mensch, Natur-Interaktionen im Oberen Mittelrheintal - Komplexitätstheoretische Ansätze am Beispiel des Weinbaus

Kulturlandschaften als Ausdruck einer über viele Jahrhunderte währenden intensiven Interaktion zwischen Menschen und der sie umgebenden natürlichen Umwelt, sind ein traditionelles Forschungsobjekt der Geographie. Mensch/Natur-Interaktionen führen zu Veränderungen der natürlichen Umwelt, indem Menschen Landschaften kultivieren und modifizieren. Die Mensch/Natur-Interaktionen im Weinbau sind intensiv rückgekoppelt, Veränderungen der natürlichen Umwelt wirken auf die in den Kulturlandschaften lebenden und wirtschaftenden Winzer zurück und beeinflussen deren weiteres Handeln, was wiederum Einfluss auf die Entwicklung der gesamten Weinbau-Kulturlandschaft hat. Kulturlandschaft wird aus diesem Grund als ein heterogenes Wirkungsgefüge sozialer und natürlicher Elemente konzeptionalisiert, an dessen Entwicklung soziale und natürliche Elemente gleichzeitig und wechselseitig beteiligt sind. Grundlegend für die vorliegende Arbeit ist die Überzeugung, dass sich Kulturlandschaften durch Mensch/Natur-Interaktionen permanent neu organisieren und nie in einen Gleichgewichtszustand geraten, sondern sich ständig weiterentwickeln und wandeln. Die Komplexitätstheorie bietet hierfür die geeignete theoretische Grundlage. Sie richtet ihren Fokus auf die Entwicklung und den Wandel von Systemen und sucht dabei nach den Funktionsweisen von Systemzusammenhängen, um ein Verständnis für das Gesamtsystemverhalten von nicht-linearen dynamischen Systemen zu erreichen. Auf der Grundlage der Komplexitätstheorie wird ein Untersuchungsschema entwickelt, dass es ermöglich, die sozio-ökonomischen und raum-strukturellen Veränderungsprozesse in der Kulturlandschaftsentwicklung als sich wechselseitig beeinflussenden Systemzusammenhang zu erfassen. Die Rekonstruktion von Entwicklungsphasen, die Analysen von raum-strukturellen Mustern und Akteurskonstellationen sowie die Identifikation von Bifurkationspunkten in der Systemgeschichte sind dabei von übergeordneter Bedeutung. Durch die Untersuchung sowohl der physisch-räumlichen als auch der sozio-ökonomischen Dimension der Kulturlandschaftsentwicklung im Weinbau des Oberen Mittelrheintals soll ein Beitrag für die geographische Erforschung von Mensch/Natur-Interaktionen im Schnittstellenbereich von Physischer Geographie und Humangeographie geleistet werden. Die Anwendung des Untersuchungsschemas erfolgt auf den Weinbau im Oberen Mittelrheintal. Das Anbaugebiet ist seit vielen Jahrzehnten einem starken Rückgang an Weinbaubetrieben und Rebfläche unterworfen. Die rückläufigen Entwicklungen seit 1950 verliefen dabei nicht linear, sondern differenzierten das System in unterschiedliche Entwicklungspfade aus. Die Betriebsstrukturen und die Rahmenbedingungen im Weinbau veränderten sich grundlegend, was sichtbare Spuren in der Kulturlandschaft hinterließ. Dies zu rekonstruieren, zu analysieren und die zu verschiedenen Phasen der Entwicklung bedeutenden externen und internen Einflussfaktoren zu identifizieren, soll dazu beitragen, ein tief greifendes Verständnis für das selbstorganisierte Systemverhalten zu generieren und darauf basierende Handlungsoptionen für zukünftige Eingriffe in die Systementwicklung aufzuzeigen

Neurally adjusted ventilatory assist in patients with critical illness-associated polyneuromyopathy

Diaphragmatic electrical activity (EA(di)), reflecting respiratory drive, and its feedback control might be impaired in critical illness-associated polyneuromyopathy (CIPM). We aimed to evaluate whether titration and prolonged application of neurally adjusted ventilatory assist (NAVA), which delivers pressure (P (aw)) in proportion to EA(di), is feasible in CIPM patients.

A robust reference signal generator for synchronized ventricular assist devices

Ventricular assist devices (VADs) are blood pumps that offer an option to support the circulation of patients with severe heart failure. Since a failing heart has a remaining pump function, its interaction with the VAD influences the hemodynamics. Ideally, the heart's action is taken into account for actuating the device such that the device is synchronized to the natural cardiac cycle. To realize this in practice, a reliable real-time algorithm for the automatic synchronization of the VAD to the heart rate is required. This paper defines the tasks such an algorithm needs to fulfill: the automatic detection of irregular heart beats and the feedback control of the phase shift between the systolic phases of the heart and the assist device. We demonstrate a possible solution to these problems and analyze its performance in two steps. First, the algorithm is tested using the MIT-BIH arrhythmia database. Second, the algorithm is implemented in a controller for a pulsatile and a continuous-flow VAD. These devices are connected to a hybrid mock circulation where three test scenarios are evaluated. The proposed algorithm ensures a reliable synchronization of the VAD to the heart cycle, while being insensitive to irregularities in the heart rate.

Cis-acting elements and developmental regulators govern toxin gene expression in Bacillus anthracis

Expression of the structural genes for the anthrax toxin proteins is coordinately controlled by host-related signals such as elevated CO2 , and the trans-acting positive regulator, AtxA. No specific binding of AtxA to the toxin gene promoters has been demonstrated and no sequence-based similarities are apparent in the promoter regions of toxin genes. We hypothesized that the toxin genes possess common structural features that are required for positive regulation. To test this hypothesis, I performed an extensive characterization of the toxin gene promoters. I determined the minimal sequences required for atxA-mediated toxin gene expression and compared these sequences for structural similarities. In silico modeling and in vitro experiments indicated significant curvature within these regions. Random mutagenesis revealed that point mutations associated with reduced transcriptional activity, mostly mapped to areas of high curvature. This work enabled the identification of two potential cis-acting elements implicated in AtxA-mediated regulation of the toxin genes. In addition to the growth condition requirements and AtxA, toxin gene expression is under growth phase regulation. The transition state regulator AbrB represses atxA expression to influence toxin synthesis. Here I report that toxin gene expression also requires sigH, a gene encoding the RNA polymerase sigma factor associated with development in B. subtilis. In the well-studied B. subtilis system, σH is part of a feedback control pathway that involves AbrB and the major response regulator of sporulation initiation, Spo0A. My data indicate that in B. anthracis, regulatory relationships exist between these developmental regulators and atxA . Interestingly, during growth in toxin-inducing conditions, sigH and abrB expression deviates from that described for B. subtilis, affecting expression of the atxA gene. These findings, combined with previous observations, suggest that the steady state level of atxA expression is critical for optimal toxin gene transcription. I propose a model whereby, under toxin-inducing conditions, control of toxin gene expression is fine-tuned by the independent effects of the developmental regulators on the expression of atxA . The growth condition-dependent changes in expression of these regulators may be crucial for the correct timing and uninterrupted expression of the toxin genes during infection. ^

Geochemical analysis of nine sections from the Nama Group, Namibia

The first appearance of skeletal metazoans in the late Ediacaran (~550 million years ago; Ma) has been linked to the widespread development of oxygenated oceanic conditions, but a precise spatial and temporal reconstruction of their evolution has not been resolved. Here we consider the evolution of ocean chemistry from ~550 to ~541 Ma across shelf-to-basin transects in the Zaris and Witputs Sub-Basins of the Nama Group, Namibia. New carbon isotope data capture the final stages of the Shuram/Wonoka deep negative C-isotope excursion, and these are complemented with a reconstruction of water column redox dynamics utilising Fe-S-C systematics and the distribution of skeletal and soft-bodied metazoans. Combined, these inter-basinal datasets provide insight into the potential role of ocean redox chemistry during this pivotal interval of major biological innovation. The strongly negative d13C values in the lower parts of the sections reflect both a secular, global change in the C-isotopic composition of Ediacaran seawater, as well as the influence of 'local' basinal effects as shown by the most negative d13C values occurring in the transition from distal to proximal ramp settings. Critical, though, is that the transition to positive d13C values postdates the appearance of calcified metazoans, indicating that the onset of biomineralization did not occur under post-excursion conditions. Significantly, we find that anoxic and ferruginous deeper water column conditions were prevalent during and after the transition to positive d13C that marks the end of the Shuram/Wonoka excursion. Thus, if the C isotope trend reflects the transition to global-scale oxygenation in the aftermath of the oxidation of a large-scale, isotopically light organic carbon pool, it was not sufficient to fully oxygenate the deep ocean. Both sub-basins reveal highly dynamic redox structures, where shallow, inner ramp settings experienced transient oxygenation. Anoxic conditions were caused either by episodic upwelling of deeper anoxic waters or higher rates of productivity. These settings supported short-lived and monospecific skeletal metazoan communities. By contrast, microbial (thrombolite) reefs, found in deeper inner- and mid-ramp settings, supported more biodiverse communities with complex ecologies and large skeletal metazoans. These long-lived reef communities, as well as Ediacaran soft-bodied biotas, are found particularly within transgressive systems, where oxygenation was persistent. We suggest that a mid-ramp position enabled physical ventilation mechanisms for shallow water column oxygenation to operate during flooding and transgressive sea-level rise. Our data support a prominent role for oxygen, and for stable oxygenated conditions in particular, in controlling both the distribution and ecology of Ediacaran skeletal metazoan communities.