Yes it is somewhat arbitrary, but text slides should have 40 words or less, including the title. It is impossible for the audience to listen to you and read your slide with more than 40 words. As a result they will do both poorly.
During my class on presentation techniques it is not uncommon for students to generate all text slides with 70 or 80 words. We edit them down to 40 words without losing content.
Perhaps the key is to recognize that you don't need complete sentences or paragraphs on a text slide. The words on your slide are cues for you to talk about various points. You can amplify each point with your voice. Use your pointer to highlight the bullet point you want to talk about and add the detail with your voice. In other words the text slide is like a table of contents for the audience with much of the detail in your oral presentation.
No cheating! While 40 is a somewhat arbitrary number, every presentation creator needs this discipline.
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