For a PhD candidate that has done a fine job in their PhD, what are the common mistakes to avoid in a defense session?
How the dissertation defense works varies enormously between fields, countries, universities, and departments. Any guidance has to take into account what the expectations are.
For example, in the defenses I'm familiar with, there are no common mistakes at all. In fact, I'm not sure I've ever seen anyone do anything worth calling a mistake. Here, the defense is basically a formality: you don't reach that point unless your advisor and readers are happy with your dissertation and confident you will easily pass, and in practice nobody fails unless something goes dreadfully wrong. That can happen (I know of one case in which a serious mistake was uncovered at the last minute, which was certainly embarrassing to both the candidate and the committee), but I've never actually been present when someone failed. So if someone from my department asked me about common mistakes they should avoid, I'd tell them not to worry about it: making it to the defense is the hard part, and then passing it is really straightforward. It's more of a celebratory ritual than an actual test.
But of course this depends on how the defense is handled in your department.
Let me turn the question around and answer what I want a PhD candidate to demonstrate during a defense. There are in reality only 3 things:
Demonstrate mastery of the subject, i.e., understand where the problem you researched comes from, why it is relevant, what others have already done, and why their approaches are insufficient.
That you have made a creative contribution to resolving the question you worked on. The creative contribution may really have been only a single idea or insight, grounded in an understanding of the field. In reality, you will have likely developed it jointly with your adviser.
That you have followed up this idea with diligent, comprehensive work that explored the potential and consequences of the idea.
If you cover these three topics, you will be on the good side.
Now for the common mistakes: Among the bad or difficult PhD defenses I have sat through -- of which unfortunately there were a number -- the candidate typically covered only one or two of the areas above. I can think of defenses where a candidate demonstrated good, diligent work, but was almost entirely unaware of the scientific background of the work. I can also think of defenses where the candidate understood the field well, but all of the work was essentially applying existing methods to problems to which it had previously been applied. And I can think of defenses where the candidate had had an idea, but whether the idea was worthwhile pursuing or not had not been explored in any kind of detail through follow-up work. In other words, I've seen PhD candidates make all kinds of mistakes. In reality, I think that the problem was in all of these cases not with the defense, but with the candidate, as usually becomes apparent during the Q&A session at the end.
IMO, the most common is being defensive (perhaps encouraged by the name of the ritual). For instance if a member asks "Why did you use Smith's procedure...", and the candidate starts backtracking and apologizing, saying "I suppose I could have used Jones' procedure, that would have been better", when the question simply asks you to explain why you used Smith's procedure.
In the end, the person in the room who probably knows most about the work done is the candidate.
- Don't get nervous
- Make sure your slides cover what you did, in sufficient detail. Don't overdo it, your time is limited. Make sure (e.g. by rehearsing) you fit in the allotted time. Might have to only show a slice for this, select it carefully.
- When asked about stuff outside your direct work, it can be better to say you don't know than to try to come up with an explanation on the spot.
- Check possible questions (within reason!) and have answers ready
I have been to multiple defenses, some common/general mistakes:
Not knowing why they use certain methods as opposed to others. Meaning that you can answer what you are doing and how the formula works, but you can't answer why not use X method instead.
Not knowing the specifics of certain methods/formulas and their reasons. Ex: we take X from Y, and you can't answer why you can/should take X from Z. more realistically: we take a measurement from the center, why not the top or bottom?
Charts with too little information, and trying to make connections that "are there" in most datasets, but doesn't imply they will be in yours. Don't create curved lines if you are missing dots.
Not knowing what is the importance of whatever it is that you are doing. Can you answer the question: "How is this useful for someone else?"
Answers in the following form: why are you using "matlab?", you: because my advisor said so. Expected response: because my advisor has plenty of experience with it and we know it can solve the problem better than other software.
The worst mistake you can make is to insist on defending on a certain date, over your advisor's objections. (I have seen this happen.)
If I may be forgiven a small anecdote: My aunt was asked a question in her defense (physics) that she didn't know how to answer. She said to the questioner, "Perhaps a sketch will help make this clear," and attempted to draw some coordinate axes... which didn't come out straight... and after several erasures and retries, the questioner got fed up and said, "It's okay, I know you know it, let's move on."
A common mistake to avoid is getting too nervous or defensive, and trying to cover up the limits of your knowledge. The committee will try to explore the limits of your knowledge and keep pressing for answers to increasingly difficult questions until you say "I don't know." Be expecting this. It's OK to not know the answer, especially when the question is not core to your thesis. If you can identify why a question is not core to your thesis, and articulate that clearly, you get bonus points for that (as long as you do it tactfully and don't imply that it's a bad question). Also, depending on the committee, they may be looking for a better answer like "I don't know, but for X reason(s) I suspect the answer might be Y and I would go about finding the answer by Z." If the question is core to your thesis, offer to find out the answer and get back to them (especially if they're on your committee).
The better your presentation, the more questions you're likely to get that try to expand outward into territory you haven't prepared for. Take that as a good sign!