Fault-tolerant routing in peer-to-peer systems

We consider the problem of designing an overlay network and routing mechanism that permits finding resources efficiently in a peer-to-peer system. We argue that many existing approaches to this problem can be modeled as the construction of a random graph embedded in a metric space whose points represent resource identifiers, where the probability of a connection between two nodes depends only on the distance between them in the metric space. We study the performance of a peer-to-peer system where nodes are embedded at grid points in a simple metric space: a one-dimensional real line. We prove upper and lower bounds on the message complexity of locating particular resources in such a system, under a variety of assumptions about failures of either nodes or the connections between them. Our lower bounds in particular show that the use of inverse power-law distributions in routing, as suggested by Kleinberg (1999), is close to optimal. We also give efficient heuristics to dynamically maintain such a system as new nodes arrive and old nodes depart. Finally, we give experimental results that suggest promising directions for future work.Comment: Full version of PODC 2002 paper. New version corrects missing conditioning in Lemma 9 and some related details in the proof of Theorem 10, with no changes to main result

Language and Learning in Ethiopia

This thesis examines the relationship between language of instruction policy and educational outcomes in Ethiopia. In 1994 Ethiopia introduced a mother tongue education policy which marked a move away from Amharic-only instruction, to the use of multiple local languages in primary schooling. This thesis investigates three key dimensions of this policy: (i) whether there is an advantage to being a ‘mother tongue learner’ in terms of learning outcomes; (ii) whether there are inequalities in learning progress between students learning in different languages of instruction, and if so, why; and (iii) whether the use of multiple mother tongues for school instruction can ensure access to essential languages of wider communication, and if not, with what implications. The mixed methods analysis finds that (i) there is an advantage to being a ‘mother tongue learner’ in Amharic language classes, but this advantage disappears when other indicators of educational experience are taken into account, and varies between linguistic environments/ contexts; (ii) that stakeholders support the use of mother tongue for reasons that relate both to pedagogy, and to the assertion of ethnolinguistic identity, emphasising the nonlearning-related benefits of the policy; (iii) that between-language of instruction inequality of learning outcomes are evident, with students learning in many of the newly introduced languages of instruction making less progress in mathematics than their counterparts in Amharic language of instruction classrooms; (iv) that these between-language inequalities in learning outcomes may be explained by variation in literate environments and linguistic development and standardisation, as well as heterogeneity of school quality between different linguistic communities; and (v) that stakeholders perceive important inequalities in opportunities to acquire languages of wider communication (Amharic and English) between students learning in different languages of instruction, raising important concerns about the extent to which the policy can translate into social and economic opportunity and mobility for all

"There are too many, but never enough": qualitative case study investigating routine coding of clinical information in depression.

We sought to understand how clinical information relating to the management of depression is routinely coded in different clinical settings and the perspectives of and implications for different stakeholders with a view to understanding how these may be aligned