http://hdl.handle.net/2440/14759 Wed, 22 May 2013 10:02:46 GMT 2013-05-22T10:02:46Z http://hdl.handle.net/2440/77940 Title: The role of friends in adolescent overweight and weight-related behaviors: a social network perspective. Author: de la Haye, Kayla Abstract: This thesis examined processes that may be driving the clustering and proposed “spread” of obesity amongst adolescent friends and within their wider friendship networks (Fowler & Christakis, 2008; Halliday & Kwak, 2009). Specifically, the aims were to determine 1) if similarities in weight status amongst friends were explained, at least to some extent, by their body mass indexes (BMIs) assimilating over time, and 2) whether this was underpinned by friends’ influence on obesity-related behaviors. Findings from two cross-sectional studies and one longitudinal study are presented in four papers, of which one has been published and the remaining three submitted for publication. Paper 1 examined associations between adolescents’ BMI and their school-based friendships longitudinally (N = 156), using stochastic actor-oriented models (SAOM) for social networks (Snijders, Steglich, & Schweinberger, 2007). Weight status was found to play a significant role in adolescents’ friendship choices, with overweight youth often marginalized by, and segregated from, their nonoverweight peers. Although there was a trend for friends’ BMIs to assimilate over the 16-month study, this effect was not statistically significant. Similarities in BMI amongst friends were therefore explained by friendship choices rather than “contagion” effects. The conditions under which overweight youth are marginalized by their peers were further explored in Paper 2. This study looked at the role of school classroom norms favoring healthiness, and specifically the norms endorsed by high-status students, in weightrelated marginalization amongst pre-adolescents. In this cross-sectional study (N = 503), exponential random graph models (ERGM) (Robins, Pattison, Kalish, & Lusher, 2007) were used to test for associations between weight status and friendships in school classes with weak versus strong health norms. Overweight students, and particularly overweight girls, were found to be marginalized by their peers in classes with strong health norms; however overweight youth were well integrated in classrooms lacking clear norms on healthiness. The results suggest that local norms may impact the relevance of attributes like weight status to adolescent friendships and thus the prevalence of weight-based stigma in peer groups, providing some useful insights for future interventions. Whether weight-related health behaviors clustered and spread amongst adolescent friends was investigated in the final two papers. The first, cross-sectional, study (Paper 3) looked at several obesity-related behaviors within three adolescent friendship networks (N = 385) and tested for similarities amongst friends using ERGMs. The strongest evidence of behavioral similarity was found for organized physical activity (PA); therefore the final paper (Paper 4) longitudinally examined the social processes driving this association using SAOMs (N = 378). Similarities in PA amongst friends over their first year of high school were found to be explained by friendship selection and influence: adolescents were likely to befriend peers whose attitudes towards PA, and engagement in PA, were similar to their own; and adolescents subsequently emulated their friends’ behaviors so that friends’ participation in PA became increasingly alike. Friends’ influence on PA was not found to be mediated via adolescents’ beliefs about PA, including their perceptions of peer norms, suggesting that this influence process was less internalized than some health behavior theories would suggest (e.g., Ajzen, 1991). As a whole, the studies presented in this thesis suggest that the clustering of overweight in adolescent friendship networks is initially driven by processes of weight-based friendship selection and the marginalization of overweight adolescents by their peers. Excess weight was not found to be contagious in the short term, and longer studies applying similar methods are needed. Nonetheless, some obesity-related behaviors were found to cluster in friendship networks, and for PA this was partially explained by adolescents adopting their friends’ behaviors. Friends’ influence on adolescent PA, and potentially other obesity-related behaviors, is a plausible mechanism that could result in the contagion of obesity in the longer-term. Intervening in, and potentially harnessing these social processes, provides a means to foster peer contexts that encourage healthy behaviors and help to reduce young people’s obesity risk in future. Fri, 31 Dec 2010 13:30:00 GMT http://hdl.handle.net/2440/77940 2010-12-31T13:30:00Z http://hdl.handle.net/2440/77855 Title: Performance evaluation of measurement algorithms used in IEDs. Author: Ibrahim, Mohammad Nizam Abstract: Many Intelligent Electronic Devices (IEDs) are available for the protection of power systems. These IEDs use a series of mathematical algorithms for fault detection and execute various protection functions. The first and essential mathematical algorithm of any IED is the measurement algorithm. The aim of the measurement algorithm is to estimate the fundamental frequency component (phasor) of input current and voltage signals. Most protection algorithms use the estimated phasor for their executions. The most important factors for the successful use of the protection algorithms in IEDs are accuracy and speed of the phasor estimation by the measurement algorithms. A fault in a power system produces step changes in the current and voltage phasors recorded by IEDs as well as a variety of nuisance signals. The nuisance signals introduce significant input distortions to measurement algorithms. Measurement algorithms that estimate the fundamental frequency phasor component from the distorted input signals produce some errors. Different measurement algorithms produce different amounts of error. This is because their design is based on different approaches with different assumptions that result in different performance in the presence of nuisance signals. It is important to evaluate the performance of measurement algorithms in the presence of nuisance signals. The evaluation is to ensure that measurement algorithms estimate the fundamental frequency component at the required design accuracy and speed. The result of the performance evaluation can be used to select appropriate measurement algorithms for specific protection applications. However, the parameters of nuisance signals are uncertain due to their dependence on unpredictable factors such as fault location and fault impedance. Thus, a methodology for the evaluation of measurement algorithm performance should take into account the uncertainty of the parameters of nuisance signals. The traditional method of evaluating the performance of measurement algorithms is based on the local sensitivity method using a linear function approximation at a nominal point. The local sensitivity method varies only a single nuisance parameter (factor) while other factors are fixed at their nominal values. The studied factor is varied to observe errors in the output of the measurement algorithm. Such an approach, however, does not provide the overall performance of measurement algorithms. Besides, varying the single factor does not represent realistic scenarios. This thesis proposes a new methodology to evaluate the performance of measurement algorithms implemented in IEDs. The proposed methodology uses the global uncertainty and sensitivity analysis method. In this method, all factors representing nuisance components are varied simultaneously. Uncertainty analysis measures the uncertainty in output of the measurement algorithm due to the uncertainty of input factors. Sensitivity analysis measures the contribution of all factors and their interactions to output uncertainty. In general, the global uncertainty and sensitivity method that is based on the Monte Carlo approach requires extensive evaluations. Its implementation can be prohibitive, particularly in practical testing, because the number of factors is large. Thus, a two-stage methodology with a significantly smaller number of evaluations is used. The first-stage is the use of the Morris method as a preliminary (screening of factors) sensitivity analysis and the second-stage is the implementation of the Extended Fourier Amplitude Sensitivity Test (EFAST) technique for comprehensive global uncertainty and sensitivity analysis. A single evaluation involves one run of the IED injection test which can take a few minutes. Thus, it is justifiable to search for the methodology that is uses the smaller number of evaluations. The proposed methodology contributes to an automated testing method integrating ATP/EMTP, MATLAB and SIMLAB programs as well as the injection test facility. The ATP/EMTP program is used to generate fault test scenarios. The MATLAB program is used to model elements of the IED to calculate performance indices on the output of measurement algorithms and automatically control the process of extensive evaluations (simulations). The main role of the SIMLAB is to analyze the uncertainty and sensitivity of the measurement algorithms outputs. The proposed methodology has been demonstrated by evaluating the performance of a known measurement algorithm in simulation and an unknown measurement algorithm of a commercial IED (SEL-421). The methodology has been successfully performed in the simulation as well as in practical testing. The results of the analysis indicate that the performance is typically most sensitive to a few parameters out of many possible factors. These important parameters should then be the focus of research for the optimization of measurement algorithms. Sat, 31 Dec 2011 13:30:00 GMT http://hdl.handle.net/2440/77855 2011-12-31T13:30:00Z http://hdl.handle.net/2440/77835 Title: Signal processing and optimisation of MIMO radar. Author: Balzan, Luke Anthony Abstract: This thesis presents a background to and a series of interesting and novel results for a particular proposed model for narrowband MIMO radar systems. The proposed model is both novel and unique, comprising closely-spaced antenna arrays that transmit orthogonally-coded waveforms, and can be seen as a logical extension to existing models for conventional single-input, single-output radar systems. Signal processing and optimisation is performed on the proposed system with a view to yield enhanced estimation and tracking performance. The proposed signal and likelihood estimation models have been thoroughly defined, with a number of important approximations and simplifications to the models gained through the use and exploitation of orthogonally-coded waveforms. All approximations and assumptions have been justified through the use of simulated examples. The Cramer-Rao bound for the models is derived and verified as correct through the use of simulated data. Through comparison of the Cramer-Rao bound to statistical estimation variances obtained through extensive simulations, the proposed models are shown to be efficient, thereby demonstrating the validity of the bound to be used as performance metric for optimisation. With the knowledge that the proposed MIMO radar system is efficient, the Cramer-Rao bound is used as a measure for estimation performance optimisation. The bound is seen to be dependent on the choice of orthogonally-coded waveforms used in the MIMO radar system, and by framing the selection of codes as a convex optimisation problem, codes can be chosen to minimise the Cramer-Rao bound, and since the system has been shown to be efficient, this also reduces estimate variance, thus improving the estimation performance of the system. This optimisation problem has been examined and simulated extensively, with simulated data substantiating the claims of performance improvement. Finally, this thesis explores the idea of tracking for MIMO radar. A Kalman filter based tracker is proposed and simulated for the MIMO radar system. Extending the convex optimisation scheme discussed above, a similar optimisation problem is formed for the case of MIMO radar tracking. The optimisation problem has been simulated to select orthogonal codes for transmitting based on the predicted target motion, obtained from the Kalman filter tracker. By basing the optimisation on the predicted tracker outputs, an action-perception cycle for MIMO radar is established, where the system is able to adapt to its surroundings based on it’s current and predicted view of the environment. Simulations have been used to observe the performance improvements of implementing the optimisation scheme, and thereby showing the action-perception cycle for MIMO radar at work. Sat, 31 Dec 2011 13:30:00 GMT http://hdl.handle.net/2440/77835 2011-12-31T13:30:00Z http://hdl.handle.net/2440/77827 Title: Effects of legume growth and residue decomposition on growth and phosphorus uptake in following wheat. Author: Mat Hassan, Hasnuri. Abstract: In phosphorus (P) deficient soils, several legumes have been shown to mobilise less labile P pools and to have a greater capacity to take up P than cereals. In conditions where N was not limiting, some legumes can increase the growth and P uptake of the following cereals which may be related to P mobilisation by the legumes. There is little information about the size of various soil P pools in the rhizosphere of legumes in soil fertilised with P although P fertiliser is often added to legumes to improve N₂ fixation. The aims of this study were to (i) compare the growth, P uptake and the concentration of rhizosphere soil P pools of different grain legumes, (ii) compare the decomposition rate of grain legume and wheat residues, and (iii) determine the effect of legume pre-crops and residue addition on growth, P uptake and concentrations of rhizosphere P pools of the following wheat. A series of plant growth experiments were carried out in a glasshouse to compare the growth of the different grain legumes and wheat and the concentrations of P pools of the rhizosphere soil. The soil pH determines the dominant P forms, therefore, two soils which were low in available P and contrasting pH (a loamy sand soil pH 8.8 and a sandy loam pH 5.4) were used in separate experiments to which soluble P was added to ensure good plant growth. Additionally, another experiment was conducted in the alkaline soil with lower P supply. Nodulated chickpea (Cicer arietinum L.), faba bean (Vicia faba L.), white lupin (Lupinus albus L.), yellow lupin (Lupinus luteus L.) narrow-leafed lupin (Lupinus angustifolius L.) and wheat were grown until maturity. Plant dry weight and P uptake were measured, sequential P fractionation was employed to determine the concentrations of P pools in the rhizosphere of the legumes and wheat. Irrespective of soil pH and P supply, growth and P uptake were greatest in faba bean whereas the less labile P pools were most strongly depleted in the rhizosphere of white lupin despite its lower growth and P uptake compared to faba bean. In the alkaline soil with high P supply, compared to the unplanted control soil, the depletion of labile pools (resin P and NaHCO₃) were greater in the rhizosphere of faba bean whereas in the alkaline soil with low P supply and the acidic soil, white lupin depleted most of the labile pools more strongly than the other legumes. An incubation study was carried out to compare the decomposition rate and the available N and P concentrations after addition of the legume and wheat residues. Shoots, roots and the combination of shoots and roots of wheat, faba bean, chickpea and white lupin were mixed into the loamy sand soil. The decomposition rate was measured over 42 days by determining soil CO₂ release and the concentrations of available P and N in the soil were measured on days 0 and 42. Chickpea shoot residue decomposed faster than the other residues. Compared to the control soil without residue addition, resin P concentration was increased with legume residue addition but not with wheat residue addition. Inorganic N was increased significantly with addition of faba bean and white lupin residues compared to the un-amended control whereas wheat residue addition had no effect. In order to differentiate between the effect of the legume pre-crop alone and that of legume pre-crop and their residue on the following wheat, soil grown with legumes from which rootand shoot residues were removed or added back were planted with wheat. Growth, P uptake and concentrations of rhizosphere P pools of the following wheat were measured. Generally, growth was greater in wheat grown in the previously unplanted soil than in the pre-cropped soils. Among the pre-crops, in the alkaline and acidic soils with high P supply, the growth of the following wheat was greater in legume pre-crop soil without residue than with residue addition. The reverse was true for plant P concentration in the alkaline soil whereas in the acidic soil, plant P concentration was similar among the treatments. Varying results with residue addition on the growth of following wheat were observed in the alkaline soil with low P supply, but residue addition consistently increased wheat P concentration. In the loamy sand (pH 8.8) with high P supply, regardless of the pre-crops, wheat depleted the less labile residual P, NaOH-Pi and particularly NaOH-Po, whereas in the sandy loam (pH 5.4), the depletion was greatest in resin P. Similarly, in the loamy sand soil with low P supply, wheat after legumes depleted labile and less labile pools more than wheat after wheat. Generally, the addition of pre-crop residues increased the size of organic P pools in the rhizosphere of wheat grown in pre-crop soils. The results of this study showed that in the alkaline loamy sand, among the legumes only those with the greatest depletion of either labile or less labile pools (faba bean at high P and white lupin at low P supply) enhanced the growth of the following wheat. At high P supply, the pre-crop faba bean with greatest depletion of labile pools resulted in a greater depletion of less labile pools by the following wheat than the other legumes. At low P supply, the pre-crop white lupin with greatest depletion of labile and less labile pools induced a greater depletion of the less labile pools in the rhizosphere of wheat. On the other hand, in the acidic sandy loam, the legumes with the greatest depletion of most pools (labile and less labile) did not increase the growth of the following wheat compared to legumes with little depletion. Furthermore, the addition of legume pre-crop residues increased the concentration of organic P pools in the rhizosphere of the following wheat compared to pre-crop alone but generally decreased wheat growth. Sat, 31 Dec 2011 13:30:00 GMT http://hdl.handle.net/2440/77827 2011-12-31T13:30:00Z