How to defend your PhD

This relates to PhD defenses in computer science / computational linguistics in Germany! The customs of your department/country/scientific discipline may differ.

Towards the end of their PhD journey, PhD students are experts on their research topic and probably already well acquainted with how to create posters about their research papers and with how to give an effective research talk that mostly covers one topic. What they have often rarely done, however, is giving a coherent talk summarizing all of their contributions. Moreover, PhD defense talks constitute a talk genre in their own, differing from both conference presentations and typical conference keynotes or invited talks. To provide some guidances, I am summarizing some expectations and tips in this post.

Key Objectives

The objective of a conference paper talk is make the audience curious about your work. The objective of an invited talk or a keynote are (in my opinion) to educate and entertain. So what is the goal of a PhD defense? Do you have to specifically defend what you’ve worked on? The ideas that you proposed? Yes and no. Plus, in natural language processing (NLP) or computer science (CS) more broadly, most content of your dissertation will have been peer-reviewed before publication at conferences or in journals beforehand, so it is unlikely that this part will be questioned. The goal is somewhat broader.

• Demonstrate Experience: Your goal is to prove that you have become a competent and independent researcher in your field.

• Demonstrate Awareness of Research Landscape: Often, PhD defense presentations only focus on the research bits conducted by the PhD candidate. Of course, that is an important part, but it is also important to precisely locate that bit in the research landscape. Do not forget to explain what the state of the art was when you started each bit of research and what the gap was that your approach/method/idea/results solve.

• Present your work: Typically, a PhD defense is between 30 and 60 minutes, so naturally, there is not sufficient time to explain everything you have worked on. Of course, you need to give an overview, but ideally, choose two publications (three if your PhD defense is a bit longer) for which you provide a deep dive.

Audience

In contrast to the conference presentation that you have probably given, the audience of your defense typically has a somewhat broader scientific background. This means that your presentation should explain everything in a way that at least every committee member can make sense of it. The examiners of your written thesis will have read your dissertation in detail. However, there are likely additional examiners for the oral exam which may not have read the dissertation in detail. Plus, you also want to make the rest of the audience happy. So explain everything in accessible and general terms, but be specific and go a bit more into detail when you deep-dive into those 2-3 papers.

Structure

• Start with explanations of the topic that you trying to solve, and the respective state-of-the-art.
• What is the motivation for working on this topic? Which problems will it solve? Think broadly. What are relationships to established subfields or research areas in NLP/CL?
• Create a slide that visualizes which topics/problems you have worked on and how they relate. In the image below, you can see examples from my own defense and from Stefan Grünewald’s (copyright for his slide: him and Bosch).

• Mention the publications explicitly, drop the citations (in a unified short format) whenever they fit. I typically use blue color for own citations and black for other relevant citations.
• Most important, you need to convey what your contributions are. You know what counts as contributions because you already listed them explicitly in the introduction sections of your papers and in your PhD dissertation. This is analogous. Make it more entertaining than showing a list. The contributions do not necessarily have to occur as one list on a slide. You can explain them along the presentation whenever they fit and summarize in the end.
• Use (animated) diagrams to explain your methods and results (as usual).
• Describe implications of your work and which opportunities for future work you see, in particular those opened up by your research.

Preparation, Time Management, Feedback, and Dry Runs

Making PhD defense slides takes time. Plan for around 2 weeks structuring and editing time, and make sure you have appointments with people helping you. Have a dry run with your team or with experienced researchers, e.g., PostDocs at your department. Personally, I am highly grateful to my PhD advisor Prof. Dr. Manfred Pinkal and to Dr. Stefan Thater, whose last-minute feedback was extremely helpful and resulted in an evening of last-minute updates on my side but also a much improved presentation!

Attend as may PhD defenses at your department as you can! See what you like/dislike, what went well, what did not go so well. Plus, there is often free food. :)

Discussion

A large part of the PhD defense is typically the discussion. There is absolutely no need to be afraid of the discussion. You are the absolute expert about your topic in the room, and you have just spent at least three years engaging actively in your research community. You know what is going on! Demonstrate your reflection about the research area by discussing with other experts what you think matters. Be aware that some questions may be a bit more high-level and test your ability to generalize and draw connections also on the meta-level. Answer both in a general and in a specific way! You will be surprised how easy and interesting the discussion will feel. Often, only the members of the committee may ask questions; sometimes also everyone in the room who is a PD (Privatdozent) or professor.

Celebration !!

I was lucky enough to do my PhD at the CoLi department of Saarland University (now called Language Science and Technology). This department really celebrated PhD defenses. Everyone went. There was a formal handshake and congratulations, and drinks and food afterwards. The fellow PhD students always created a hat with silly items on it. The recent graduate has to guess what the items stand for, and only if they guessed everyone correctly, they were allowed to put on the hat. It was an awesome time, and worthy celebration of x years of hard work!

In the pictures, you can see

1. me celebrating with my PhD advisor Manfred Pinkal, a linguist and computer scientist and pioneer in computational linguistics
2. me guessing what the items on the hat mean
3. me having guessed everything correctly and wearing the hat

Enjoy your PhD defense!