There's an episode of the classic television series Johnny Quest ("Treasure of the Temple," I think) in which Dr. Benton Quest, in some far off exotic locale, comes across an unfortunate local who has been staked and tied, spread-eagled, to the ground. The local speaks urgently to Dr. Quest in his native tongue.
The brilliant scientist says, slowly, deliberately, and loudly, "WHO... DID THIS... TO YOU?"
After all, everyone understands English if you just say it loud enough.
A lot of professors, though, do to their students much the same thing as Dr. Quest did to the native. They think if they can just say things more clearly, their student will understand. I've had many conversations with colleagues who bemoan their students' poor performance on an exam, and say, "But I told them..." They think that if they can become a better lecturer, explain things more clearly, that students will understand better and retain more.
For instance, "Students didn't do well on this material, but I really didn't stress its importance enough. So next I'll be very clear to tell them this is a big part of the exam, and they need to study it."
Unfortunately, this may be completely misguided.
When I was at the Society for Integrative and Comparative Biology meeting back in January, there was a session on education in evolution. Craig Nelson talked about data that has been gathering in physics education for a good while now. The data from physics education indicates there is no such thing as a good lecturer.
Students learn about the same amount regardless of the lecturer when the traditional lecture format is used.
The thing that actually starts to improve student scores and understanding are not better lectures. Better lectures are like saying, "WHO... DID THIS... TO YOU?" to the native even slower and louder. Instead of staying with traditional lectures, instructors need to incorporate other techniques that allow and require students to develop some of the knowledge themselves. Having students listen just doesn't cut it.
Yet very few professors seem willing to deviate from standard lectures and rebuild their classes from the ground up. Even to take advantage of something fairly simple, like clickers.