Presents research on and discusses the redundancy effect, consistent with a dual-channel theory of multimedia learning in which adding on-screen text can overload the visual information-processing channel, causing learners to split their visual attention between two sources. In research, lower transfer performance also occurred when interesting but irrelevant details were added to the video clips within or before the presentation. (Contains 33 references, 3 tables, and 2 appendixes.) (GCP)