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I will soon defend my PhD in social science in Sweden. I want to ask what questions the evaluation committee will ask me during the defense? If they ask me some questions outside my thesis, what shall I do? Is it enough only know well my own research? What quality I need to successfully defend my PhD?

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    Have you not gone to other people's defenses before? Have you asked your advisor? – xLeitix Mar 30 '15 at 11:19
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    I don't recall ever hearing any restrictions on what I can ask in a thesis defense. Ordinarily, I ask about things in the thesis, prerequisites for things in the thesis, and things that the candidate happens to mention during the defense. I could, in principle, ask a completely off-topic question, to which the candidate might well reply "I have no idea"; presumably, such an answer wouldn't damage the outcome of the defense (unless the other 4 examiners agree with my craziness). – Andreas Blass Mar 30 '15 at 13:37
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    Whatever field you study and whatever type of presentation you give, always be prepared for questions asking you why you made certain decisions. It won't hurt preparing for them and it might actually improve your presentation if you hadn't thought about it before. – Mast Mar 30 '15 at 13:49
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    Obligatory XKCD: xkcd.com/1403 – Corey Ogburn Mar 30 '15 at 18:17
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    I was asked "who is Maud Menten?" (see 1910's at famouscanadianwomen.com/famous%20firsts/…) Since I used the Michaelis-Menten equation in my work, it was borderline relevant. Luckily there is a plaque to her on the U of T campus, so I knew both the equation half of the answer and the trivia question half. That was the only question that examiner asked. Seriously, know your thesis well and ask your supervisor if you can expect anything unusual, and you'll be ok. They don't generally let you defend if they don't think you're ready. – Kate Gregory Mar 30 '15 at 18:32
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Generally speaking, PhD defenses come in two flavors, and it is not possible to say a priori which one yours will fall into:

  • A real thesis "defense" focuses on the thesis, and all questions will relate to the scope of the research work. In this case, knowing your own work and its context well enough should be sufficient, but it is of course possible that somebody asks you why you used method A rather than B. In this case, saying that you don't know B may not fly as an answer. However, you should not get a completely disconnected question just to test your general subject area knowledge.
  • A "Rigorosum" (dict.leo translated this for me to "doctoral viva", no idea if this is a well-known term) is a general final exam for a doctoral candidate. Typically, this will also include questions about the disseration, but anybody in the exam commission is free to ask about other topics in the field as well. Typically, the questions are getting "easier" (more high-level) the farther they are away from the topic of the dissertation. The claim here is that a fresh PhD should have both, depth and breadth in her/his knowledge.

There are a number of ways to figure out which class your defense will fall into:

  • Attend other defenses. Typically, PhD defenses are public, and it is highly recommended that any PhD candidate should visit a few before her/his own, to get to know the procedure and get a feel for what the defense talk is supposed to look like.
  • Ask your advisor. The advisor should obviously know what the requirements of the defense are, exactly.
  • Look it up in the programme description. Usually, the programme description will contain exam regulations for the defense, and this should describe the exam procedure and who is supposed to ask what kind of questions. This approach has two dangers: (1) you may misunderstand what the exam regulations are saying - as they are more legalese text, you may misunderstand the gist of it, and (2) the actual exam may happen differently than specified - there is no telling whether the exam regulation exactly captures the real spirit of the defense. Hence, you should really also talk to your advisor and/or attend other defenses as well.
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    Typically, there's a non-public part of the defense after the public part where the general public is asked to leave and the committee can grill the candidate in private. That's where the interesting/hard/potentially embarrassing questions are likely to come. This is harder to prepare for and benefits from asking your advisor and recent other students for guidance. – Bill Barth Mar 30 '15 at 13:24
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    @BillBarth Ok, I was not aware of that. In all universities that I was aware of, everything needs to be public to have traceability of decisions. – xLeitix Mar 30 '15 at 16:42
  • "Viva" (short for viva voce) is the term used in the UK (and some other countries?), I'm not sure if use of this term corresponds to your two categories or not. At my UK university you are expected to answer questions generally on your field as well as your thesis. – MJeffryes Mar 30 '15 at 16:48
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    In addition to that, vivas in the UK are typically behind closed doors in their entirety - only two(ish) examiners and the candidate, with even the advisor being absent. – E.P. Mar 30 '15 at 22:28
  • good answer. i would like to add that the OP should look into their supervisor's reviews of the thesis. in some countries/unis these are provided beforehand. they contain the main criticism and these will come up also in the defense. the defense talk itself should already address them. – henning -- reinstate Monica Jul 12 '15 at 16:11
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Some types of questions that often come up:

  1. Questions about some detail of your work. The point of such questions is often not so much the correctness of this small part of your dissertation, but rather to get you to demonstrate your ability to explain what you've done. This also helps to balance out the presentation, which typically covered the whole thesis in much less detail. For example, in mathematics you might be asked to go over the proof of one of your theorems in detail. Since you've spent more time working on these details than any of the examiners, and assuming that the examiner hasn't spotted a substantial error, you should be in a good position to answer such a question. Don't panic and assume that you've made some huge mistake, but rather go through the material carefully and convince the examiner that you're correct.

  2. What are the broader implications of your work? This is actually a hard question for most students to answer because they've been so focused on the details of their thesis that they may not have taken time to see how it fits into the broader picture of progress in their discipline. This will also be an important question when you eventually interview for faculty positions.

  3. Can this method be applied to some other problem Y? You may be asked how the techniques used in your dissertation could be applied to a different problem. If the answer is that the technique doesn't extend in that direction, then you should be prepared to explain why not.

  4. Questions about what exactly is new in your work. You should have been extremely careful in writing your dissertation to properly cite earlier work and distinguish your new contributions from that earlier work. If this isn't clear to the examiners, you may be asked to clarify.

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I attended about a dozen PhD defenses and recently defended my own PhD. Based on my (very limited) experiences, questions can be roughly categorized into two types:

  1. Knowledge: the first set of questions or statements are usually to test your knowledge on your topic, the related work and whatever you have written down in your thesis and/or papers. If you have done all the work, these are mostly fairly easy to answer and are your way to demonstrate the awesome work you have done. These questions are to clarify, justify and frame your work related to others, in order to get a solid understanding of your contribution. Examples:

    • Can you explain what you mean with #concept you introduce#?
    • Why did you categorize or describe #your concept# in this way?
    • I believe #your topic# relates to #other guys theory# in such a way, which introduces an interesting contradiction. Can you comment or elaborate on this?
    • What is the main limitation of your work, and how could you address it?
  2. Reasoning: committee members may also ask provocative or even harsh questions to see how strong you believe in your work but also whether you can transcend your topic and reason on a higher level. These questions or statements can be directly related to your topic but can also be more general. These types of questions are usually given to see how well the candidate performs when put under pressure and when they are questioned about things beyond their thesis. These can be interpreted as "nasty" questions, but remember that you can turn the questions around to your advantage. Examples:

    • Why do you believe this is science? What is science?
    • You spend #x# years researching this? Why did you choose the topic and why should we and the community care about it?
    • I don't see the contribution in your work, can you explain?
    • Why do you think you deserve a PhD?
    • Why did you choose this specific application domain?

Again, this is based on my personal limited experience, but it applied to my PhD defense as well.

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You do not have much influence on the questions from the committee. If your PhD studies were more or less normal and you got that far, everyone wants you to succeed. They will not ask anything incredible (if they are relatively normal as well).

As for the public questions, my experiences are :

  • the friends you gave questions to. You have to choose the ones who have good reflexes to jump up and down when time for public questions come. You also have to train the "what a surprising question look"

  • the odd colleague who decided to ask a question about something related to your field, but far enough for you to have no idea. You sweat a lot, visualize him hanging on a tree and hope for the best. Best is to answer whatever you know on the subject and everyone acts happy. If he persists then you ask him what is his view on the subject so that he sits down ashamed mumbling "I have no idea"

  • finally the 173 years old professor who absolutely wants you to discuss his article from 1952. You thank for the question and talk about the article from 2012 someone wrote on a related subject.

Good luck.

(events described above actually happened to me in the course of my 2 hours defence)

  • Even though I will not ever have to go though a PhD defense anymore, it would have been great to understand the downvotes. – WoJ Jul 12 '15 at 17:59

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