Student Number : 9802206M - MA dissertation - School of Psychology - Faculty of Humanities / Before this study took place, the social psychology perspective of Attribution theory was yet to be fully utilised in South African research within the maintenance error landscape. Attributional approaches see the person on the street operating like a scientist, obtaining information from his or her social surroundings and discerning the causes and consequences of ongoing behavioural and environmental events (Harvey et.al.,1976). It is very possible that due to the unique South African socio-political and economic landscape, strongly influenced by Apartheid, new combinations of known and unknown error attributions are at play, that are unique to this landscape and have not yet been studied or uncovered. Thus, a better understanding of the South African landscape, through a study such as this, could have serious cost benefits to maintenance companies, benefits to staff in terms of reduced risk of injury, as well as form the basis of improved policies, procedures and equipment. Twenty-five team leaders and 125 minor maintenance staff at a South African Aircraft Maintenance Company formed the population group from which the sample for this study was drawn. Within each group, 5 individuals were interviewed on a personal basis. Further, for each group, one focus group was carried out consisting of two and four individuals respectively. The individuals who participated in the focus groups were different to those who participated in the interviews. In total 28 percent of team leaders participated in the study and 7 percent of maintenance staff, which calculates to just over 10 percent of minor maintenance employees at the organisation involved. The Qualitative data acquired through this in-depth interview and focus group discussion process, and subsequent transcription was coded and analysed using Thematic Content Analysis. Content analysis is a research technique for making replicable and valid inferences from data to their context (Krippendorf, 1980). The discussion of primary error attributions comparing maintenance staff and team leaders, focussed on the predominant primary error attribution theme and related attributions under the descriptor Organisational Culture which included both the dimensions of employee motivation and managerial culture. Finally, results of secondary error attributions comparing maintenance staff and team leaders raised the discussion around the theme, Tools and Equipment. This research is an exploratory study that brings together the field of attribution theory and maintenance error. Its main strength is that it provides a theoretical framework, upon which is based a methodology that explores the primary and secondary error attributions made by employees for maintenance errors in their work environment. In other words, it is felt that this methodology can be implemented in a range of maintenance environments to unearth the error attributions of staff in that environment. Information such as this is very beneficial to companies and organisations in their planning, strategising, problem solving and general organisational development.

Diagnostic errors cause significant patient harm and occur among 15 percent of all clinical diagnoses, but research has yet to effectively target, prevent, and mitigate diagnostic errors from occurring. So far, literature has examined how diagnostician decision-makers perform and reach a clinical diagnosis individually. However, the impact of team-based activities on diagnosis is unknown. The purpose of this study is to describe provider perception on how providers come together as a team in order to complete a clinical diagnosis. As a qualitative descriptive study with overtones of grounded theory, 18 semi-structured interviews of medical intensive care unit providers were audio-recorded, transcribed, and coded generating themes of diagnostic teamwork structure and functioning. Diagnostic teams are described using themes of inter-professional and intra-professional teamwork among roles with and without diagnostic team identity. Novel approaches to diagnostic error research, practice implications for current providers, and applications provided for improving education and team training. By providing preliminary insights on the role of teamwork in diagnostic decision-making, this study may assist future studies that improve diagnostic teamwork and prevent diagnostic errors.

The main purpose of this thesis is to compare the performance of the score test and the likelihood ratio test by computing type I errors and type II errors when the tests are applied to the geometric distribution and inflated binomial distribution. We first derive test statistics of the score test and the likelihood ratio test for both distributions. We then use the software package R to perform a simulation to study the behavior of the two tests. We derive the R codes to calculate the two types of error for each distribution. We create lots of samples to approximate the likelihood of type I error and type II error by changing the values of parameters. In the first chapter, we discuss the motivation behind the work presented in this thesis. Also, we introduce the definitions used throughout the paper. In the second chapter, we derive test statistics for the likelihood ratio test and the score test for the geometric distribution. For the score test, we consider the score test using both the observed information matrix and the expected information matrix, and obtain the score test statistic zO and zI . Chapter 3 discusses the likelihood ratio test and the score test for the inflated binomial distribution. The main parameter of interest is w, so p is a nuisance parameter in this case. We derive the likelihood ratio test statistics and the score test statistics to test w. In both tests, the nuisance parameter p is estimated using maximum likelihood estimator pˆ. We also consider the score test using both the observed and the expected information matrices. Chapter 4 focuses on the score test in the inflated binomial distribution. We generate data to follow the zero inflated binomial distribution by using the package R. We plot the graph of the ratio of the two score test statistics for the sample data, zI /zO , in terms of different values of n0, the number of zero values in the sample. In chapter 5, we discuss and compare the use of the score test using two types of information matrices. We perform a simulation study to estimate the two types of errors when applying the test to the geometric distribution and the inflated binomial distribution. We plot the percentage of the two errors by fixing different parameters, such as the probability p and the number of trials m. Finally, we conclude by briefly summarizing the results in chapter 6.

Simulation methods are used when closed-form solutions do not exist. An interesting simulation method that has been widely used in many scientific fields is the Monte Carlo method. Not only it is a simple technique that enables to estimate the quantity of interest, but it can also provide relevant information about the value to be estimated through its confidence interval. However, the use of classical Monte Carlo method is not a reasonable choice when dealing with rare event probabilities. In fact, very small probabilities require a huge number of simulation runs, and thus, the computational time of the simulation increases significantly. This observation lies behind the main motivation of the present work. In this thesis, we propose efficient importance sampling estimators to evaluate rare events probabilities. In the first part of the thesis, we consider a variety of turbulence regimes, and we study the outage probability of free-space optics communication systems under a generalized pointing error model with both a nonzero boresight component and different horizontal and vertical jitter effects. More specifically, we use an importance sampling approach,based on the exponential twisting technique to offer fast and accurate results. We also show that our approach extends to the multihop scenario. In the second part of the thesis, we are interested in assessing the outage probability achieved by some diversity techniques over generalized fading channels. In many circumstances, this is related to the difficult question of analyzing the statistics of the sum of random variables. More specifically, we propose robust importance sampling schemes that efficiently evaluate the outage probability of diversity receivers over Gamma-Gamma, α − µ, κ − µ, and η − µ fading channels. The proposed estimators satisfy the well-known bounded relative error criterion for both maximum ratio combining and equal gain combining cases. We show the accuracy and the efficiency of our approach compared to naive Monte Carlo via some selected numerical simulations in both case studies. In the last part of this thesis, we propose efficient importance sampling estimators for the left tail of positive Gaussian quadratic forms in both real and complex settings. We show that these estimators possess the bounded relative error property. These estimators are then used to estimate the outage probability of maximum ratio combining diversity receivers over correlated Nakagami-m or correlated Rician fading channels

Several traffic characterization studies have been performed on wireless LANs with the main objective of realizing good and accurate models of the errors in the wireless channel. These models have been extended to model the effect of errors on higher layer protocols, mainly at the data link layer. However, no prior work has been done to study the application level characteristics of MPEG-4 video traffic over 802.11b wireless networks. In this thesis a traffic characterization study of MPEG-4 video traffic over IEEE 802.11b wireless LANs with the main goal of building a tool for software testing is performed. Using two freely available tools to send and receive real-time streams and collect and analyze traces, MPEG-4 encoded video frames are sent over a 11 Mbps, 802.11b wireless LAN to characterize the errors in the channel and the effect of those errors on the quality of the movie. The results of this traffic characterization were modeled using ARTA (Auto Regressive-To-Anything) software. These modeled characteristics were then used to build a tool that generates synthetic traffic emulating real wireless network scenario. The tool emulates the error length and error free length characteristics of the wireless network for the MPEG-4 video traffic using the corresponding modeled characteristics generated by ARTA. The tool can be used by software developers to test their MPEG-4 streaming media applications without the need of the real infrastructure. The tool can also be trained and extended to support testing of any streaming media applications.

Efficient receiver designs for cooperative communication networks are becoming increasingly important. In previous work, cooperative networks communicated with the use of L relays. As the receiver is constrained, channel shortening and reduced-rank techniques were employed to design the preprocessing matrix that reduces the length of the received vector from L to U. In the first part of the work, a receiver structure is proposed which combines our proposed threshold selection criteria with the joint iterative optimization (JIO) algorithm that is based on the mean square error (MSE). Our receiver assists in determining the optimal U. Furthermore, this receiver provides the freedom to choose U for each frame depending on the tolerable difference allowed for MSE. Our study and simulation results show that by choosing an appropriate threshold, it is possible to gain in terms of complexity savings while having no or minimal effect on the BER performance of the system. Furthermore, the effect of channel estimation on the performance of the cooperative system is investigated. In the second part of the work, a joint preprocessor-based detector for cooperative communication networks is proposed for one-way and two-way relaying. This joint preprocessor-based detector operates on the principles of minimizing the symbol error rate (SER) instead of minimizing MSE. For a realistic assessment, pilot symbols are used to estimate the channel. From our simulations, it can be observed that our proposed detector achieves the same SER performance as that of the maximum likelihood (ML) detector with all participating relays. Additionally, our detector outperforms selection combining (SC), channel shortening (CS) scheme and reduced-rank techniques when using the same U. Finally, our proposed scheme has the lowest computational complexity.

This thesis is concerned primarily with the optimization of the current regulation in bi-directional ac-dc power converters through the use of appropriate current control methods. Following a review into prior current control technology, current control methods which attempt to achieve Zero Average Current Error (ZACE) in each switching period are presented. A ZACE controlled converter offers independent real and reactive power flow control with negligible low order current harmonics, a relatively narrow switching frequency band, and relative immunity to power circuit parameter variations, including DC link or AC line voltage harmonics. ZACE and other desirable characteristics in a current control method are discussed. The single phase ac and dc ripple current is characterized.Two new types of ZACE current control techniques for directly controlling the inductor current in switched power converters are introduced in this thesis together with variations for certain applications. Slope-generated hysteresis (SGH), the first to be developed, is a hysteresis method which uses the slopes of the current error signal alone to generate a hysteresis band which will result in a fixed switching frequency. Slope-generated hysteresis-clock (SGHC) is presented as an alternative with a dual clock to force a narrow switching frequency band.Ramptime current control is the second type of ZACE current control presented. Developed as an improvement over SGH, ramptime produces ZACE in each switching period by using the timing of a previous switching instant relative to the coincident previous current error signal excursion time to determine each switching instant. The digital current error polarity signal is the only variable input required to produce a pwm output.Variations of ramptime current control are also presented. Polarized ramptime is a subset of ramptime which maintains a narrow ++ / switching frequency band despite switching delays. Dual ramptime is the final enhancement of ramptime where two polarized ramptime regulators are used together to provide the appropriate choice between full-bridge and half-bridge switching in a single phase current controlled full-bridge voltage source inverter with the ac ripple current minimized without compromising the transient response. Using this technique, excellent fidelity and a narrow switching frequency band are demonstrated.The ZACE current control techniques are applied to a three phase voltage source inverter. A "standing phase" system of control for a three wire, three phase inverter is chosen over individual phase control since only two current regulators are required to control two decoupled current error signals, and the effective switching frequency is reduced by one third.The new ZACE methods are found to compare favourably in simulation to existing linear and hysteresis type current control techniques. SGH current control has equivalent fidelity to any other hysteresis control in delivering the reference current waveform, but is prone to noise in the hysteresis band determination requiring filtering. This, combined with the effect of switching delays compromises the narrowness of the switching frequency band. SGHC current control is also prone to noise in the generation of the hysteresis band, and results in a decrease in the fidelity of reproduction of the reference waveform. Ramptime current control is a robust technique, largely immune to power circuit parameter and voltage variances, with good fidelity and a relatively narrow switching frequency band. Polarized ramptime current control is shown to produce excellent fidelity with a narrow switching frequency band.The operation of the ZACE methods in single and three phase prototype converters is demonstrated. A field installation of a ++ / grid-connected ramptime current controlled converter is shown to source 20 kW of real power onto the grid from a photovoltaic array with a maximum power point tracking control, while independently providing grid voltage support through reactive power control.The effect of the synchronization of the current regulators on the ac and dc current ripple are presented. Synchronized polarized ramptime regulators are shown to produce the minimum ripple current in simulation and in the prototype operation.ZACE current control techniques, and ramptime and polarized ramptime in particular, are presented as a significant contribution to the control of current in power electronic converters.

Physical-Layer Network Coding (PNC) is a promising technique that has great potentials for improving the achievable data rates of end-to-end flows through higher packet transmission rates, thereby increasing the overall network throughput. In this thesis, we study the performance of the PNC transmission techniques for unidirectional end-to-end flows in multi-hop wireless networks, and compare it with that of the traditional transmission techniques. We first derive the bit-error rate (BER) that the PNC transmission technique achieves. Then, using the derived BER, we evaluate and quantify the achievable network throughput under both the PNC transmission technique and the traditional technique, where the network throughput is measured as the aggregate/sum of all end-to-end flows' achievable data rates in the wireless network. Using extensive simulations, we show that PNC increases the overall achievable end-to-end flow throughput in multi-hop wireless networks, especially under medium to high signal-to-noise ratios. / Graduation date: 2012

High order multistep methods, run at constant stepsize, are very effective for integrating the Newtonian solar system for extended periods of time. I have studied the stability and error growth of these methods when applied to harmonic oscillators and two-body systems like the Sun-Jupiter pair. I have also tried to design better multistep integrators than the traditional Stormer and Cowell methods, and I have found a few interesting ones.

Thesis (M.A. (English))--North-West University, Potchefstroom Campus, 2006.