The secret to remembering material long-term is not to cram and over-learn but rather to periodically review what you’ve studied. That’s according to Doug Rohrer and Harold Pashler who have identified an intriguing relationship between how long to leave it before returning to previously studied material, and the ultimate duration for which you want to remember it for.
The technical definition for ‘over-learning’ is any time you spend continuing to study material which you have already mastered. So, for example, once you’ve correctly recalled a list of French vocab without any errors, any additional time you immediately spend learning that vocab is over-learning. The evidence shows that time spent over-learning is only beneficial over the short-term. For example, one study found over-learning was advantageous when tested a week later, but not when tested four weeks later.
According to Rohrer and Pashler, if your aim is long-term retention, time spent over-learning would be better spent reviewing material at a later date. Just how much later depends on how long you want to remember the material for. Their research suggests the optimal time to review material is after a period which is 10 to 30 per cent of the time for which you want to remember it for. Reviewing too soon, or too near the later test will be associated with poorer learning. For instance, one study that tested retention after ten days (always measured from the second ‘review’ session) found that from a range of 5 minutes to 14 days, the optimal time for review was after one day. Another study that looked at retention over 6 months, found the optimal time for reviewing material was one month.
The researchers say their observations have implications for the design of textbooks. For example, most maths books tend to end each chapter with numerous problems prompting over-learning of that chapter’s material. It would be more effective if a variety of problems were posed at the end of each chapter so that students were continually reviewing material studied in earlier chapters.
Rohrer, D. & Pashler, H. (2007). Increasing retention time without increasing study time. Current Directions in Psychological Science, 16, 183-186.