The "speech suppression effect of PowerPoint"
No substitute for simply talking to the audience

University of Munich sample regular slide
Researchers at Ludwig-Maximilians-Universität München, led by Professor Christoph Wecker, tested two types of slides—what they called “regular” and “concise” slides—versus just talking to the audience. The researchers concluded that the retention of oral information was significantly lower during lectures that used regular slides compared to lectures without slides.

Professor Wecker calls this a speech suppression effect. If you buy into the argument that presentation value comes from what the presenter says, suppressing spoken information is counterproductive. Significantly less makes it from tank to trough, which negatively impacts communication effectiveness.

However, if the audience loses a large portion of what’s said while slides are shown, is there an offsetting gain to using slides? If slides take away from communication effectiveness in one area, do they add value somewhere else?

Dr. Wecker’s team determined the answer to both questions is an unequivocal no. “It is remarkable, however, that this ‘suppressive’ effect of regular slides,” they said, “could not be demonstrated to be the downside of a trade-off in favour of the retention of information on slides.”

When presentations use what the researchers call regular slides (of precisely the same density—the same number of letters and words—as the slide above), audiences lose huge portions of what is said. And the communication effectiveness lost by showing slides is not regained anywhere else, leading to a net negative impact of using slides.

I changed the words on this slide into gibberish, but this was the exact density of a slide in the study. With about thirty-five words each, Professor Wecker’s regular slides are almost identical to the six-by-six guideline (six lines of text with six words per line) that many presenters have adopted as a rule of thumb. Dr. Wecker demonstrated that the six-by-six approach significantly decreases a presenter’s ability to move information to the long-term memory of the audience, decreasing communication effectiveness.
University of Munich study shows sample concise slide
The researchers also tested what they called concise slides. This test, using slides identical in density to the slide shown at the right, contained six slides (four fewer than the presentation with standard slides): a title slide, a structural slide, and four additional slides. None of the concise slides contained more than seven lines (excluding the heading) or five bullet points. The example on this page has twelve words.

The researchers also used “black” slides between the regular slides, attempting to create somewhat of a hybrid between standard and conversational presentations. However, while the hybrid improved communication outcomes when compared to standard slides, nothing worked better than removing slides from view and simply talking to the audience. Bullet points, even those as skimpy as what’s shown on both of these slides, overload working memory.

What does this mean for you? You have a choice. You can continue to show bullet points, quotes, sentences and paragraphs to the audience. If you do, the message is one hundred percent clear: your communication effectiveness will be negatively impacted. People will forget what you’ve said as soon as you’re done.

Alternatively, you can choose to set yourself and your ideas apart by focusing on having a conversation with the audience. Use notes to keep yourself on track and on time when you deliver your presentations. But don’t show your notes to the audience.

When you need a visual to aid understanding, use it. But when it’s no longer needed, remove it from view and carry on the conversation.