In one of his own experiments, Dr. Roediger and Jeffrey Karpicke, also of Washington University, had college students study science passages from a reading comprehension test, in short study periods. When students studied the same material twice, in back-to-back sessions, they did very well on a test given immediately afterward, then began to forget the material.
But if they studied the passage just once and did a practice test in the second session, they did very well on one test two days later, and another given a week later.