You might be making your audience multi-task
If you're one of those presenters who packs your slides with content, wanting to put each word that you're saying on the slide, your strategy is likely back-firing.
Research has shown us that when your audience tries to both listen to you speak and read each word on your slide, they are likely not processing that information as well as if they were just focused on listening to you or just reading.
If an audience member read what you're saying in a report, focusing strictly on that, they could process the information properly and understand it fully. Similarly, if you told them your idea, passionately and clearly, while sitting across for them, and they could listen to you intently, they'd also absorb it properly. However, in many PowerPoint presentations, the audience is asked to do both.
What's the evidence around this issue?
The "cognitive load theory" was developed by researchers from the University of New South Wales in the 1980s and one of the findings was that it is more difficult for people to process information if it is coming at them both verbally and in written form at the same time.
And yet, we know that if our slides are packed, our audience will try to read them while while listening for the simple reason that they don't want to miss something important.
So what should you do in your PowerPoint presentations?
Don't make your audience work so hard to process your message! That's a lose-lose.
Follow the following practices:
Say everything you need to say verbally, so the audience can focus on you
Only include your main main points or summaries in your slides to reinforce your points
If you can, use visuals (relevant, high quality, grabbing photos) as images are far more likely to be remembered, and when they're associated with your point or message, that point or message will be remembered to
If you need, give your audience a leave-behind report with all the important data or detailed points. If you have this, tell them ahead of time, so they don't feel they have to write down everything you say and can, again, focus on you as you speak.