Of the many tips, recommendations, and best practices for delivering a presentation, the one rule that is not up for discussion and not meant to be bent or broken, ever, is this one: Don't go over time. Nothing you can say or do will be worth the inconvenience that overrunning your allotted time would bring to your audience, the speaker who comes after you, the organisers, and the event as a whole.
Not going over time is mostly the speaker's responsibility, but the organisers of an event also share some responsibility in making this work.
First of all, as an organiser, you need to clearly communicate to your speakers how the schedule of your event works and how long you expect them to talk. If each talk starts on the hour, some speakers will - inadvertently or due to inexperience - assume that they have an hour for their presentation. But you need to leave some room for speakers to switch over, for people to switch rooms (if you have several sessions running in parallel) and of course leave some time for questions. For a one-hour slot, that gives the speaker something like 45 minutes for the talk, 10 minutes for questions, and 5 minutes for the switchover; and that's an idealistic scenario.
Which brings me to my other point: Organisers may need to enforce sticking to the scheduled times or at least gently nudge speakers and remind them not to go over time. As interesting as the talk or the discussion might be - if they overrun their schedule, it will cause all sorts of problems for the individuals in the room and for the schedule of the entire event.
The tried and tested way to do that is to have a moderator in each session. He or she can then signal to the speaker when time's up, can help moderate questions and bring some additional authority in helping to end on time. Even experienced speakers tend to get carried away or forget the time - they're happy to talk about their topic and if you don't stop them, they'll happily talk about it for hours without even noticing that they're running over their allotted time. The audience, on the other hand, may want it to end but they are usually too polite to just get up and leave, especially if they haven't had a chance to applaud yet. Applause helps marking the end of the talk, or at least the part where only the speaker is talking. It's somewhat easier to leave during Q&A (unless you're stuck in a row of seats) but still feels awkward and impolite.
If you don't have moderators for your sessions, you should at the very least have someone who looks after each speaker before they begin, to welcome them and in case they need help with the technical setup. That's a good opportunity to double-check with the speaker that he or she is aware of the schedule.
When you do the retrospective after your event, take things like keeping within the allotted time into account when you rate speakers. Going over time could be over-enthusiasm on the part of the speaker but it could also be a sign of bad preparation. Check back with your speakers and give them feedback (not just when they went over time, of course). They may not even be aware of your concerns; if you don't tell them, they will never know.
Clear communication before and after the event helps to clarify expectations and requirements, like the schedule. Have a moderator in each room or at least have someone check with the speaker before they begin. Provide feedback for your speakers after their talk so they can improve. And don't forget to ask them for feedback on how you, as the organisers, could improve things for them.
(Photo Credit: Time's Up by Brandon Le, CC BY, from Flickr)
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