I have seen hundreds of academic presentations, the great majority with slides. Unfortunately, I would say that the most common practice is one of the worst: putting all or nearly all of what you will say on your slides. Since nearly everybody reads faster than you will talk, this gives the whole presentation a dragging sense of going over the same ground slowly and repeatedly.
Putting totally different text on your slides is in some ways even worse, since now the audience needs to follow two simultaneous narratives which may not combine perfectly or benefit from being put forward in parallel.
The approach which I use and have found successful is to make myself a Powerpoint deck with speaking notes in point form. With the slides on a screen that only I can see, I can be sure to follow my overall outline and not miss any points. It also lets the audience concentrate fully on what I am saying, without the distraction of comparing it against text on slides. Generally I think it sounds more engaging and human to turn point form text into full sentences on the fly, rather than read a speech verbatim, but if the presentation is very short it can be best to have everything written succinctly in advance and then to try to read it in a way that doesn’t sound like a recitation.
A couple of ways to have slides with fewer problems are to only include very brief summary or conclusion text, which the audience will be able to read so quickly it isn’t a distraction, or using slides more-or-less exclusively to show charts, graphs, and photos. That is what I tried to do with my recent lesson on the robotic exploration of the solar system.
These approaches do detract from the viability of a slide deck as a standalone presentation which can be understood without the accompanying speech, but I would argue that a deck meant to be used that way should have a totally different design from slides meant to support a spoken talk, whether it’s in-person or online. If the priority is the live audience, the talk should be designed to engage them in the moment and not to be a set of reference materials that would be equally comprehensible to someone who hasn’t heard the talk.
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Working for the Feds I have seen a number of horrid “decks” that try to jam as much info as possible onto slides… two books I absolutely love are Slide:ology and Resonate by Nancy Duarte… I wish people would follow her advice on slides. less is more… but as you surmise they should be meant to accompany a talk/presentation, not be a substitute for a report, but often they are designed for both.