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Note this rule also indirectly enforces the above mentioned rules about font sizes and the number of symbols that requires explanation that you may introduce. It is more useful and significant  than it may superficially appear.

Now I add two more such rules concerning content.

OR 2. Slides must be Self-explanatory to a reader – ideally your slides should be SELF EXPLANATORY to your intended audience without requiring your accompanying speech or sideline help  to  make the content clear. Note by this I do not mean that you speech  that goes with the talk is superfluous. Your talk adds  further depths  and insight. But the slides by themselves should be able to stand by themselves  at the level of comprehension  they command.

There are two types of intended audiences you are trying to reach with the content of your slides.  First,  your intended audience may be  “general audience” who wants to be educated and are interested in the societal significances of your talk. They are willing to accept  certain assertions from you without necessarily understand  the science behind them. For example, we accept that nothing can  travel faster than  the speed of light without understanding  the theory of relativity. But we find the implications of this limit on  space travel and communication fascinating (e.g., it takes  at least   4.5 years for one way travel to the nearest star system even  at the speed of light. It takes 8 hours to get a simple yes-no answer  from Pluto.)
The other type of your intended audience are experts or colleagues in your field of expertise. They are interested in “ what is the NEW idea in your talk?”. They may already be familiar with the basic scientific theory and facts of the subject of your talk.  You are there to impress them with your unique knowledge and insight not your command  of mathematics. Here again, I want your slides to be self explanatory with no  or minimal help fromyou.
This self-explanatory requirement helps making sure you  put the right content on the slides from an audience-centered  viewpoint.
Let me reference a previous blog article of mine to help illustrate partially the points here.
        Essence of Engineering (2) – An Example  精选   http://blog.sciencenet.cn/home.php?mod=space&uid=1565&do=blog&id=1057130  
The third objective rule on slide content is often forgotten and not practiced. This is
OR 3. Improvement based on audience feedback
During and after a talk you often realize what you did wrong and can be improved. But how often and how many of us do  go back to the slides and make necessary changes particularly  if you know that you are not going to give the talk again.  This step Is important because it reminds us not to make such  similar mistake next time