Say you are speaking for an hour to an audience of 100. Its just a fact of human nature that nobody in the audience is going to be paying close attention to what you are saying for more than 1/4 of the time. The other 45 minutes of the time people will be thinking, talking, or just daydreaming. You must accept this as an unavoidable constraint.
Absent any intervention on your part then you will get a randomly selected 15 minutes of attention from each member of the audience. This means that at any one point in time you will have the attention of only 1/4 of your audience or 25 out of the 100 people. The very important things you will have to say will be processed and potentially remembered by 1/4 of your audience, the same fraction that will be paying attention to the least important things you have to say.
So what you should do to prepare is ask yourself what are the three important things you have to say and you want remembered. Each of them should take you five minutes to say. Then imagine you have a sign that will flash above you which tells everyone in the audience whether now is the time to be paying close attention or now is an opportunity to doze off. With that sign you could coordinate their attention so that all of them are listening during the same 15 minutes, those 15 minutes when you will be saying your three important things.
Now you probably won’t be bringing that sign with you. But you can achieve the same effect by using the way that you stand, the way that you talk, and the style of your slides. When you are saying something important you speak slowly and loudly and you walk up and down the room and make eye contact and your slides have just one or two things on them so that they are easy to read and process.
You are telling them with your demeanor that now is the time to listen. Later, when you are saying something less important you lower your voice, go faster, stand still and read off your busy slides. You are doing these things to tell your audience that now is the time to think, talk or doodle and rest up for the next important moment.
Cheap Talk 
7 comments
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April 23, 2013 at 2:00 am
Anonymous
There is usually strong correlation between understanding something at time t and t+\delta.
April 23, 2013 at 8:49 am
jwell33
This is a great way of thinking about it. Thanks.
April 23, 2013 at 12:45 pm
luca
the 15 minutes are not really randomly selected, though. many more people pay attention at the beginning, for example. so, some of the best advice i received about my job-talk was to make sure those who pay attention for the first five minutes and for the conclusions (while sleeping the rest of the time) would want to give me the job.
April 23, 2013 at 4:40 pm
Tom P
A study “tested students on their recall of facts contained in a 20-minute presentation. While you might expect that recall of the final section of the presentation would be greatest — the part heard most recently — in fact the result was strikingly opposite. Students remembered far more of what they’d heard at the very beginning of the lecture. By the 15-minute mark, they’d mostly zoned out.”
See here:
http://ideas.time.com/2012/10/02/why-lectures-are-ineffective/
April 26, 2013 at 6:45 am
Enrique
How about randomizing one’s slides? Power point should have a “randomness” button that would mix and shuffle one’s slides randomly. That might keep both the speaker (not just the audience) more attentive, if he or she wasn’t sure what slide would come next ….
June 4, 2013 at 1:09 pm
dan s
same goes for papers (?)
July 20, 2013 at 10:45 am
Robert
If it were random, a point in time would obviously include more than one quarter the population because of overlapping segments. The first commenter correctly identified that it is indeed not random. Next, the presenter”s signal to the audience of an important slide is not credible. However, if they constrain their talk to only 15 minutes, then it would be credible. It seems, given the blog’s name, that this argument would have immediately been given.