When my students are preparing presentations for conferences, I always tell them that the first 2-3 lines are particularly critical and I want them to think carefully about how they will introduce the talk. After the first few lines, the rest of a talk is typically straightforward (data, interpretation, conclusions), but the first few lines are where you either grab the audience or you don't. This is when you lay out why the work is interesting and important, and why anyone should care about the rest of what you have to say. In fact, it's a lot like writing a proposal..
Today a student practiced his talk for me, and his first line was about the techniques and materials. I told him that that sentence could be the third or fourth line, but not the first. This discouraged him, so we discussed it for a while. It turns out that he didn't feel confident giving a cosmic and general first sentence because he has decided that everyone in the audience will know more about the topic than he does and so will know better than he does why the study is being done.
This student has been doing excellent and original research on the topic for several years. In fact, I would even say that he is the world expert on this topic. There will no doubt be some very brilliant people at his talk who have broad knowledge of related topics, but even so, he has to lay out the philosophy of the research at the very beginning of the talk. I think the people he is intimidated by are going to be intrigued by his results, and I told him so very emphatically. I told him that even if the entire audience consists of experts in his exact research topic, he still has to explain why this research is important. As the audience will not consist entirely of experts in his research topic, it is even more important to introduce the motivation of the talk at the very beginning.
I asked him how he would answer the question "Why is this research interesting and important?" if asked by (a) his mother, (b) an undergraduate science student, and (c) a so-called expert in the research topic, and he had excellent responses for each. Then he took bits and pieces of his various responses and crafted an awesome first two sentences. I think his talk is going to be really great.
8 months ago