From the excellent beyond bullets website… all about the use (and abuse) of PowerPoint…

Adding text to a screen in a multimedia presentation that is identical to the narration harms the ability of the audience to understand the information.

Removing the text increases retention, or the ability to remember the information, by 28%.

Even more significantly, removing the text increases transfer, or the ability to apply the information, by 79%.

This is according to research by the most prolific researcher in the field of educational psychology, Dr. Richard E. Mayer, in his book Multimedia Learning