I do wonder if there is a difference btwn not knowing 20% or even 50% of the material you are going to be taught in the lesson and not getting the answers correct to 20% or 50% of the questions after you have been taught the material. I do think that dds would be frustrated if they were missing that many of the questions.
I don't necessarily think that the level of instruction is inadequate b/c the child is getting most of the questions correct as long as he can continue to move on to new concepts once he has shown that he learned most of what he was just taught.
