The Principal Stratification method estimates a causal intervention effect by taking account of subjects' differences in participation, adherence or compliance. The current Principal Stratification method has been mostly used in randomized intervention trials with randomization at a single (individual) level with subjects who were randomly assigned to either intervention or control condition. However, randomized intervention trials have been conducted at group level instead of individual level in many scientific fields. This is so called "two-level randomization", where randomization is conducted at a group (second) level, above an individual level but outcome is often observed at individual level within each group. The incorrect inferences may result from the causal modeling if one only considers the compliance from individual level, but ignores it or be determine it from group level for a two-level randomized trial. The Principal Stratification method thus needs to be further developed to address this issue. To extend application of the Principal Stratification method, this research developed a new methodology for causal inferences in two-level intervention trials which principal stratification can be formed by both group level and individual level compliance. Built on the original Principal Stratification method, the new method incorporates a range of alternative methods to assess causal effects on a population when data on exposure at the group level are incomplete or limited, and are data at individual level. We use the Gatekeeper Training Trial, as a motivating example as well as for illustration. This study is focused on how to examine the intervention causal effect for schools that varied by level of adoption of the intervention program (Early-adopter vs. Later-adopter). In our case, the traditional Exclusion Restriction Assumption for Principal Stratification method is no longer hold. The results show that the intervention had a stronger impact on Later-Adopter group than Early-Adopter group for all participated schools. These impacts were larger for later trained schools than earlier trained schools. The study also shows that the intervention has a different impact on middle and high schools.

Magister Educationis - MEd / Since the inception of the University in 1960 under the Apartheid regime and up until a few years ago Afrikaans had dominated both as educational and as communication medium. However, political change - at the University and in South Africa - has brought about a change in language patterns. According to the HSRC Work Committee on Languages and Language Instruction (1981), a high percentage (40%) of Afrikaans-speaking people classified as coloured and living in the Cape Peninsula choose English as the medium of instruction and also regard it as the most important language to be learnt at schools (also 40%). As a result of this, and the enrolment of a large percentage of Xhosa speaking students, who generally prefer English to Afrikaans as an educational medium, English has become the language most commonly used both inside and outside the classroom. The homogeneity of preference has, however, not been matched by mastery of the language. For approximately 70% of all first-year students English is a second or even a third language. Though University entry is based on at least ten years of the study of English, standards of proficiency differ quite considerably as a result of inequalities in education. Because students need to improve their level of English proficiency I investigated the effectiveness of using senior students as tutor assistants in an academic development programme, the English Special Project, at UWC. This study aims at revealing that the tutor assistantships in the English Special Project can alleviate problems that occur with annual increases in student numbers under certain conditions. These are that: 1 prospective tutor assistants undergo proper tutor assistant training; 2 tutor assistants are committed to the course of academic development; 3 tutor assistants have the time available that is necessary for the task. In Chapter 1 give an insight into what this study is about, the reasons for conducting it and what my personal involvement with the ESP has been. In Chapter 2 I review some of the overseas and local literature on TA systems. In Chapter 3 I describe a case-study to evaluate the success of the TA system in the ESP with reference to academic development. I emphasize the intensive, individual care of and attention to first-year students. In an attempt to address the problem, the effectiveness of senior students as tutor assistants to assist in tutorial sessions is described. In Chapter 4 I discuss the value of TA intervention in the ESP at personal and social levels. Chapter 5 contains the general conclusions and my recommendations to improve the ESP.