Researchers in Australia are catching up to what better presenters already know:
It is more difficult to process information if it is coming at you in the written and spoken form at the same time.
Prof John Seller sez:
 “It is effective to speak to a diagram, because it presents information in a different form. But it is not effective to speak the same words that are written, because it is putting too much load on the mind and decreases your ability to understand what is being presented.”
Garr Reynolds beat him to it back in 2005, although he takes a more “I know it when I see it approach.” Reynolds’ one blog post formed the basis for every PowerPoint presentation  or every discussion about presentations I’ve had since.
Not to criticize Prof Seller or the ed writer in SMH, but…well…yeah.