How can you be sure that your textual slides are as efficient and high-impact as possible?

Here's a fast outline: 1. Try not to be messy. Your information introduction is your image.

2. Bring crazy concentration, and disentangle.

3. Adjust information height ideally.

4. Take out diversions, make information the legend!

5. Lines, bars, pies… push… pick the best fit.

6. Merge information, be as legitimate as you can be.

7. Dump the content, envision the story. We will have a great deal of fun, and take in some not really evident lessons.

It's not the ink, it's the think.

The research is the most effective way to get the most out of it

the more you research the more impactful your presentation be 

One last thing One idea per slide

Make handouts rather than filling up the slides.

Hope you will be satisfied with this answer 

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For further queries ask me anytime.

everything depends on your behaviour & experience.

It all depends on your audience. Some audiences like the slides to be one particular style, while other audiences would prefer it to be in another style.

Professionally, in my experience what I have noticed is that in the corporate sector, people prefer the slides to be clear, and brief. Elaborated as follows: 

1. Clarity is achieved through using a white background, and a black font size 16-20 (Depending on the projector resolution). 

2. The audience should focus on what you are saying; not trying too hard to read what is on the slides.

3. If it is information, keep it to a few bullet points. If it's a graph/Chart, make sure the legends are clear, and the data is visible at least up to 20 feet away. 

4. If you are using colours, use simple ones that do not strain the eyes. Try not to use too many emojis (if you are planing to use emojis). Mature audiences generally won't pay much attention to emojis/decorative templates etc; in fact, high profile audiences might not even like it.