by Robert Frost,
who teaches NASA instructors how to use PowerPoint effectively
The 7x7 Rule states that a PowerPoint slide (or any other electronic slide) should have no more than seven lines of text and no more than seven words in each of those lines.
Personally, I think that the 7x7 should be treated as a clear flag for a problematic slide and not an acceptable norm. Slides with that many lines of text are generally not good slides. They defeat the purpose of a slide, which is to visually convey the idea the presenter is talking about.
My recommendation is that a slide should depict one idea. If it has multiple ideas it should be because the intent of the slide is explicitly to compare or contrast those ideas.
If multiple ideas need to be put on one slide, they should not appear at once, but should appear when they are needed. It is a bad practice to put up information before the presenter is ready to talk about it because the audience doesn't wait.
There is a concept called the billboard rule. The billboard rule says that it should take no more than three seconds to read the text on a billboard (for obvious reasons). Nancy Duarte, an expert in presentations, recommends applying the billboard rule to presentation slides. There are obviously exceptions, such as when you want the audience to read an extended quote.
The reason it is recommended is that people read about 5 words a second. If you have 49 words (via the 7x7), the focus of the audience will be lost for at least 10 seconds as they read the text and stop listening to the speaker.