During a research presentation there will often be times when a conclusion needs to be supported by a complicated set of experimental results. For example, elucidating a mechanism may have required a series of experiments to rule out a long list of alternative processes, finally leaving only the one mechanism which was consistent with the findings. In such a situation, the listeners can easily become confused, distracted, or bored. One technique which increases the clarity of the presentation and saves time may seem counterintuitive at first glance—begin at the end.
Suppose you have a conclusion which is supported by a complicated set of evidence. You could go step by step through the evidence, ruling out alternatives along the way and presenting only the consistent explanation at the end. But along the way your listeners will be in the dark about the outcome and will not be sure where this will lead. Each new possible hypothesis will engage their attention momentarily, but only until it is discarded. On the other hand if you start at the end, everything becomes clear. You say in effect, “Here is the mechanism we favor. It is supported by the following findings.” Now the supporting data is presented, all of which is consistent with the mechanism proposed. For example, “The mechanism requires second order kinetics. Therefore we conducted the following experiment which showed that the kinetics were in fact second order.” With each finding the mechanism is more firmly placed in the listener’s minds. Alternative mechanisms are mentioned, but only briefly, since they are not consistent with the evidence.
The technique of beginning at the end also saves time. If each alternative mechanism is discussed in detail before being discarded, the presentation takes longer. Also, when the correct mechanism is finally presented, the speaker generally feels obliged to sum up the reasons why alternatives were inconsistent, in effect, presenting the evidence twice.
When I was a graduate student we were required to give a 50-minute presentation in our third year summarizing our research up to that point. My rehearsal took two hours. My advisor gave me the tip about beginning at the end when presenting mechanistic details and I made the change in my final presentation. This time I finished in less than an hour.