A lot depends on the thesis and its presentation: if the student has already explained some issues well, then evaluators won't have to ask many questions about these. Still, the most important points are: *what is new/unique about the thesis: how does it contribute to theory or methodology development or practice? *why did the student select these theories/research streams? *why did the student select this method/these methods, which shortcomings they tend to have and how they were dealt with? *how were the data collected & why in this way (e.g. why 3 people were interviewed, why these, is it enough to get 100 responses to a survey, maybe the statistics were not correct....)? *how did the results differ from previous authors' results? why? and *what should be done in this area in the future?

These are the frequents questions in the defenses I have been a member:

• What have been the significant contributions of your research?
• How could you improve your work?
• What would you do differently if you were setting out to do it today?

The rest of questions are related to the results and methodology

I agree with Dr. kamal. The most important thing is to be sure that the candidate did the work by himself, and he understand his work

Last Wednesday, I was an internal examiner to a student. I asked him 12 questions , but before that I told him, in front of  the audience, to be patient with me. During the questions, I told him that a good thesis is good for the university's reputation. He succeeded but under the condition of correcting all the mistakes found by the external examiner & me. I knew that his work was original by the many English language errors in the thesis! 

Dear @Eddie, if it was successful presentation, my attention will be on scientific contribution, ideas for future work based on thesis, possible applications and maybe some recommendations toward the publishing a research paper(s)! It would be very short one, just a few minutes.

Much more discussion is ongoing if there were a problems in presentation and communication (non skilled candidate). Sometimes, member of committee should play a role of mediator!

I upvoted the answer of Dr Nizar. A good internal and external examiners would enrich the thesis with their comments. I have not been an examiner of a thesis before, but I attended many dissertations defenses, and in few times you can realize that the examiner didn't read the thesis before!! 

I focus my questions on the problem statement, the methodology, the results and conclusion.

At the same time, I focus also on the statistical analysis as a technique to achieve the results and conclusion. The scientific contribution is an indicator of how rich and valuable is the thesis or dissertation is.

It is difficult to say what questions are to be asked in a particular case. They will  depend of each case. The questions can be variegated depending upon the strengths and weaknesses of the thesis. Some of the generic questions can be about the hypothesis/objectives,  methodology, reliability of the data, method sampling and data collection, interpretation of the data, review of literature, limitations of the thesis, and so on.

The type of questions might be of the following sort:

1. Questions that secure that the candidate knows the basic facts of the theory he is dealing.

2. Convince me that the thesis is an original contribution to science

3. convince me that the candidate constitute a nice scientific future.

4. Some questions that separate the intelligent advanced student from the ordinary middle. 

Dear @Eddie, I do recommend for reading the following article on this issue, full of experience The Dissertation Defense: We're Doing Something Right! "The defense also allows the faculty to say thank you. Working with graduate students on their dissertations provides us with interesting and challenging work. We don't just let them share—they also let us. We should think about that privilege more often than we do."

http://chronicle.com/article/The-Dissertation-Defense-/135386/

Ljubomir. Thanks a lot. I hope you won't mind if I share this to my students. Ed

Dear @Eddie, please share. It is my pleasure. That is why RG community serves for! Regards!

Something more for your students!

http://thegradstudentway.com/blog/?p=1525

Dear Ljubomir,

I found the article interesting. However it is written by an English literature, not from a mathematician. In literature the questions might be more subjective and confusing, whereas in the mathematics case the questions are clear and referring usually to core knowledge. Indeed you do not have to shoot questions at the candidate.  The way he has written his dissertation and the way he presents it, says a lot for the candidate.  Some questions of exegetical nature will help.

About Methods, Results, and particularly the implications of the thesis findings.

One question which I have asked is when the researcher used some one's ideas without acknowledging the person from whose work the idea is taken. When I brought this to notice of the lady researcher about this thing while conducting the viva voce, she started crying out of fear and guilt as she thought that she might fail in the viva voce. This was not plagiarism as such but a statement without quoting the source that was not acceptable. I just forgave her for this.

You should ensure you are thoroughly familiar with the content of your thesis and prepare for questions. Many expected or unexpected questions you may be asked, such as, “what have been the significant contributions of your research?” or “why did you choose this subject” or “what would you do differently if you were to repeat your PhD?” or “why did you take these variables?

First step is to ensure that the thesis subject is innovative, and the candidate is familiar with its content (ie he/she is the author of the thesis) for example he/she masters the listed references. Then verify the written quality of the document (the format, spelling and grammars errors) to ensure the seriousness of the candidate. The third step is to verify the exactness of the theories,  the problematic, the given solution and its novelty or originality. Then verify the experimental setup norms and exactness, the calculus validity and the results formats; and let the candidate focus on his/her challenging idea/result and  future  prospects or new questions raised from the research.

It is expected of candidate to have a clear understanding on the topic of study and how the current study fills the gap in terms of method/s used as well as subjects or either country context. In short what is uniqueness and how to explain it in terms of originality of the work of the interest.