How to prepare a technical talk

November 26, 2013 at 6:29 am Leave a comment

I used to suck at giving technical talks. I would usually confuse my audience, and often confuse myself. By the time I became a prof, I sucked a lot less. These days I enjoy giving technical talks and lectures more than non-technical ones, and my students seem to like them better as well.

So something had changed; I’d developed a process. The other day I sat down to see if I could extract what this process was. It turned out to be surprisingly formulaic, like an algorithm, so I’d like to share it with you. I’m guessing this is obvious to most professors who teach technical topics, but I hope it will be helpful to those who’re relatively new to the game.

There are three steps. They’re simple but not easy.

  1. Identify the atomic concepts
  2. Draw the dependency graph
  3. Find a topological ordering of the graph

Identify atomic concepts. The key word here is atomic. The idea is to introduce only one key concept at one time and give the audience time to internalize the concept before moving on to the next one.

This is hard for two reasons. First, concepts that seem atomic to an expert are often an amalgam of different concepts. Second, it’s audience-specific. You have to have a good mental model of which concepts are already familiar to your audience.

Draw the dependency graph. Occasionally I use a whiteboard for this, but usually it’s in my head. This is a tricky step because it’s easy to miss dependencies. When the topic I’m teaching is the design of a technical system, I ask myself questions like, “what could go wrong in this component?” and “why wasn’t this alternative design used?” This helps me flesh out the internal logic of the system in the form of a graph.

Find a topological ordering. This is just a fancy way of saying we want to order the concepts so that each concept only depends on the ones already introduced. Sometimes this is straightforward, but sometimes the dependency graph has cycles!

Of the topics I’ve taught recently, Bitcoin seems especially difficult in this regard. Each concept is bootstrapped off of the others, but somehow the system magically works when you put everything together. What I do in these cases is introduce intermediate steps that don’t exist in the actual design I’m teaching, and remove them later [1].

Think of a technical topic as a skyscraper. When it’s presented in a paper, it’s analogous to unveiling a finished building. The audience can admire it and check that it’s stable/correct (say, by verifying theorems or other technical arguments.) But just as staring at a building doesn’t help you learn how to build one, the presentation in a typical paper is all but useless for pedagogical purposes. Having dependencies between concepts is perfectly acceptable in papers, because papers are not meant to be read in a single pass.

The instructor’s role, then, is to reverse engineer how the final concept might plausibly be built up step by step. This is analogous to showing the scaffolding of the building and explaining each step in its construction. Talks and lectures, unlike papers, must necessarily have this linear form because the audience can’t keep state in their heads.

[1] This process introduces new nodes in the dependency graph and removes some edges so that it is no longer cyclic.

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I’m an associate professor of computer science at Princeton. I research (and teach) information privacy and security, and moonlight in technology policy.

This is a blog about my research on breaking data anonymization, and more broadly about information privacy, law and policy.

For an explanation of the blog title and more info, see the About page.

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