Topology-dependent density optima for efficient simultaneous network exploration

A random search process in a networked environment is governed by the time it takes to visit every node, termed the cover time. Often, a networked process does not proceed in isolation but competes with many instances of itself within the same environment. A key unanswered question is how to optimise this process: how many concurrent searchers can a topology support before the benefits of parallelism are outweighed by competition for space? Here, we introduce the searcher-averaged parallel cover time (APCT) to quantify these economies of scale. We show that the APCT of the networked symmetric exclusion process is optimised at a searcher density that is well predicted by the spectral gap. Furthermore, we find that non-equilibrium processes, realised through the addition of bias, can support significantly increased density optima. Our results suggest novel hybrid strategies of serial and parallel search for efficient information gathering in social interaction and biological transport networks.This work was supported by the EPSRC Systems Biology DTC Grant No. EP/G03706X/1 (D.B.W.), a Royal Society Wolfson Research Merit Award (R.E.B.), a Leverhulme Research Fellowship (R.E.B.), the BBSRC UK Multi-Scale Biology Network Grant No. BB/M025888/1 (R.E.B. and F.G.W.), and Trinity College, Cambridge (F.G.W.)

Reachable sets analysis in the cooperative control of pursuer vehicles

This thesis is concerned with the Pursuit-and-Evasion (PE) problem where the pursuer aims to minimize the time to capture the evader while the evader tries to prevent capture. In the problem, the evader has two advantages: a higher manoeuvrability and that the pursuer is uncertain about the evader's state. Cooperation among multiple pursuer vehicles can thus be used to overcome the evader’s advantages. The focus here is on the formulation and development of frameworks and algorithms for cooperation amongst pursuers, aiming at feasible implementation on real and autonomous vehicles. The thesis is split into Parts I and II. Part I considers the problem of capturing an evader of higher manoeuvrability in a deterministic PE game. The approach is the employment of Forward Reachable Set (FRS) analysis in the pursuers’ control. The analysis considers the coverage of the evader’s FRS, which is the set of reachable states at a future time, with the pursuer’s FRS and assumes that the chance of capturing the evader is dependent on the degree of the coverage. Using the union of multiple pursuers’ FRSs intuitively leads to more evader FRS coverage and this forms the mechanism of cooperation. A framework for cooperative control based on the FRS coverage, or FRS-based control, is proposed. Two control algorithms were developed within this framework. Part II additionally introduces the problem of evader state uncertainty due to noise and limited field-of-view of the pursuers’ sensors. A search-and-capture (SAC) problem is the result and a hybrid architecture, which includes multi-sensor estimation using the Particle Filter as well as FRS-based control, is proposed to accomplish the SAC task. The two control algorithms in Part I were tested in simulations against an optimal guidance algorithm. The results show that both algorithms yield a better performance in terms of time and miss distance. The results in Part II demonstrate the effectiveness of the hybrid architecture for the SAC task. The proposed frameworks and algorithms provide insights for the development of effective and more efficient control of pursuer vehicles and can be useful in the practical applications such as defence systems and civil law enforcement

The effect of diverse development goals on computer-based system dependability

PhD ThesisSociety's increasing dependence upon software control and information process- ing provision has demanded comparable increases in software dependability. While the existing software dependability approach has resulted in significant improve- ments, its focus is heavily aimed towards achieving software dependability via redundant fault-tolerant mechanisms built into the software artifact to provide error-control in the presence of activated faults. Less emphasis appears to have been placed upon how software dependability can also be promoted through a fault-avoidance approach in the software creation process by incorporating hu- man redundancy and diversity. In this thesis, a process intervention which can potentially improve fault-avoidance is considered. This involves the setting of diverse development goals within important generic computer-based system con- texts in order to increase detection of potentially harmful assumptions which can result in subtle systemic conflicts that can undermine the dependability of the re- sultant artifact during the early development phases of requirements, specification and design. A search theoretic simulation model is progressed and developed to capture some of the important dynamics involved. The eventual outputs of the simulation model indicate that increased fault coverage and sensitivity can be ob- tained through the setting of diverse development goals during the early phases of software development.EPSR

Global Financial Risk, Domestic Financial Access, and Unemployment Dynamics

We empirically show that after an increase in global financial risk, the response of unemployment is markedly more subdued in emerging economies (EMEs) relative to small open advanced economies (SOAEs), while the differential response of GDP and investment across the two country groups is noticeably smaller, if at all, in EMEs. A model with banking frictions, frictional unemployment, and household and firm heterogeneity in financial inclusion can help rationalize these facts. Limited financial inclusion among households is central to explaining the differ- ential response of unemployment in EMEs amid global financial risk shocks

Modelling search and stopping in interactive information retrieval

Searching for information when using a computerised retrieval system is a complex and inherently interactive process. Individuals during a search session may issue multiple queries, and examine a varying number of result summaries and documents per query. Searchers must also decide when to stop assessing content for relevance - or decide when to stop their search session altogether. Despite being such a fundamental activity, only a limited number of studies have explored stopping behaviours in detail, with a majority reporting that searchers stop because they decide that what they have found feels "good enough". Notwithstanding the limited exploration of stopping during search, the phenomenon is central to the study of Information Retrieval, playing a role in the models and measures that we employ. However, the current de facto assumption considers that searchers will examine k documents - examining up to a fixed depth. In this thesis, we examine searcher stopping behaviours under a number of different search contexts. We conduct and report on two user studies, examining how result summary lengths and a variation of search tasks and goals affect such behaviours. Interaction data from these studies are then used to ground extensive simulations of interaction, exploring a number of different stopping heuristics (operationalised as twelve stopping strategies). We consider how well the proposed strategies perform and match up with real-world stopping behaviours. As part of our contribution, we also propose the Complex Searcher Model, a high-level conceptual searcher model that encodes stopping behaviours at different points throughout the search process. Within the Complex Searcher Model, we also propose a new results page stopping decision point. From this new stopping decision point, searchers can obtain an impression of the page before deciding to enter or abandon it. Results presented and discussed demonstrate that searchers employ a range of different stopping strategies, with no strategy standing out in terms of performance and approximations offered. Stopping behaviours are clearly not fixed, but are rather adaptive in nature. This complex picture reinforces the idea that modelling stopping behaviour is difficult. However, simplistic stopping strategies do offer good performance and approximations, such as the frustration-based stopping strategy. This strategy considers a searcher's tolerance to non-relevance. We also find that combination strategies - such as those combining a searcher's satisfaction with finding relevant material, and their frustration towards observing non-relevant material - also consistently offer good approximations and performance. In addition, we also demonstrate that the inclusion of the additional stopping decision point within the Complex Searcher Model provides significant improvements to performance over our baseline implementation. It also offers improvements to the approximations of real-world searcher stopping behaviours. This work motivates a revision of how we currently model the search process and demonstrates that different stopping heuristics need to be considered within the models and measures that we use in Information Retrieval. Measures should be reformed according to the stopping behaviours of searchers. A number of potential avenues for future exploration can also be considered, such as modelling the stopping behaviours of searchers individually (rather than as a population), and to explore and consider a wider variety of different stopping heuristics under different search contexts. Despite the inherently difficult task that understanding and modelling the stopping behaviours of searchers represents, potential benefits of further exploration in this area will undoubtedly aid the searchers of future retrieval systems - with further work bringing about improved interfaces and experiences

Morphological Adaptation and Digestion in Relation to Raptor Feeding Ecology

1. An integral part of the study was to relate intestinal morphology to digestive efficiency in raptors. The source of morphology data was carcasses handed in by the public. The environmental conditions and time for which carcasses were exposed were unknown, as were the storage procedures following collection. The validity of using gut morphology data from carcass analysis was tested by assessing the extent to which small intestine length and weight in two-week old cockerels (Gallus gallus) changed under different experimental conditions of time and temperature post-mortem. Intestine weight decreased significantly with increases in time and temperature. Intestine length changed to a lesser extent and was chosen as the preferred measure when restricted to using carcass data. 2. Having determined which measure of gross gut morphology to use, data from Falconiformes and Strigiformes were used to quantify interspecific differences in small intestine length, the region of the gut responsible for food absorption. The study assessed the influence of predatory behaviour and prey type on morphological adaptations of the flight musculature and gut. Falconiform species were categorised as either 'attackers' or 'searchers' depending on the degree to which active, powered pursuit is required for prey capture. Attacking species feed predominantly on avian prey, requiring extreme agility, speed and acceleration for prey capture. Searchers feed largely on relatively slow-moving mammals and carrion. Weight minimisation is very important in terms of flight energetics and it was hypothesised that attackers would minimise the weight of internal organs which are not important for flight, such as intestinal mass. Searchers which do not require such agility and acceleration for prey capture would be expected to have longer, heavier intestines. It is further considered whether the absolute length or weight of the gut is important or whether it is the weight associated with gut contents that influences the size of the digestive tract. A skeletal body-size measure was determined to enable calculation of intestine length independent of body-size and shape differences. Attacking species were found to have a snail intestine which was up to 50% shorter than found in searchers of equivalent body- size. Strigiformes which locate prey by active flight also had intestinal tracts shorter than expected. It is hypothesised that these interspecific differences in gross gut morphology result in corresponding differences in digestive efficiency. 3. The size of the small intestine, stomach, liver, kidney and heart were compared between species and considered in relation to hunting strategy and body size for several raptor species. The extent to which these organs are affected by differences in body condition and parasite burden was examined. No relationship was found between parasite burden and intestine length. There was a strong correlation between body condition and organ size. Condition, fat content and parasite burden were shown to be related. Attacking species were found to have a small stomach and intestine for their size; searchers had large, heavy digestive organs. The more active owl species also had a lighter digestive tract. The scaling of intestine length, area and volume with body-mass was discussed. 4. It was hypothesised that the relatively long small intestine found in searchers such as the Red Kite (Milvus milvus) and Common Buzzard (Biiteo buteo) is adaptive and results in increased digestive efficiency, whereas a short digestive tract as found in the Sparrowhawk (Accipiter nisus), Peregrine Falcon (Falco peregrinus) and Goshawk (Accipiter gentilis) is selected for higher flight performance but results in reduced digestive efficiency. In order to test this hypothesis, it was necessary to find an appropriate method to measure digestion. The study aimed to test the suitability of titanium dioxide as a nutritional marker for measuring digestive efficiency in raptors. Such a method would enable a larger sample of birds to be used since it would allow the use of birds which could not be tethered under experimental conditions and those which are permanently kept in large aviaries. Birds which had been trained by falconry techniques were used to compare the use of a marker with results based on total faecal collection. Titanium dioxide is supposedly inert. However, complete recovery of the marker was not achieved and titanium dioxide was determined not to be a suitable marker for digestion studies in raptors. Total faecal collection was therefore used throughout the remainder of the study for measuring food passage and digestive efficiency. (Abstract shortened by ProQuest.)

With our own hands : research for Third World development; Canada's contribution through the International Development Research Centre, 1970-1985

French version available in IDRC Digital Library: Selon nos idées : la recherche au service du Tiers Monde; le Centre de recherches pour le développement international et la contribution du Canada, 1970-1985Spanish version available in IDRC Digital Library: Con nuestras propias manos : investigación para el desarrollo del Tercer Mundo; la contribución de Canadá mediante el Centro Internacional de Investigaciones para el Desarrollo, 1970-1985Part of the collection of government and miscellaneous documents relating to the history of IDRC