Format for defending a doctoral thesis
Every institution will have specific regulations for the thesis defence. In some countries or institutions, the convention is for thesis defences to be public events where you will give a lecture explaining your research, followed by a discussion with a panel of examiners (opponents). Both your examiners and the audience are able to ask questions.
In other countries, including the UK, the oral examination is usually conducted behind closed doors by at least two examiners, usually with at least one being from another institution (external examiner) and an expert in your topic of research. In the UK the supervisor does not participate in the viva, but may be allowed to observe. Sometimes someone from your own institution is appointed as an independent chair. Although it is now becoming more common for the candidate to have an opportunity to give a public lecture in UK institutions, this does not form part of the examination and may or may not be attended by the examiners.
