What it is
You might know this: you did not manage to start early and the exam is in 2 days. Now you need to cram everything you have to learn in these two days and spent hours in the library. Distributed practice is the exact opposite of this: start studying long enough before exams to be able to study the material several times for a shorter amount of time. For example, five hours spread out over two weeks instead of five hours in one go.
How to do it
You will have to plan ahead and overcome the common tendency to procrastinate: plan a little bit of time every day to review information. This strategy is easily applicable in PBL: after you review information from your most recent class, make sure to go back and study important older information. Take the last 15 min of a study session to review materials from the week before. Start early and set aside a bit of time every day, even if your exams are far away. Of course, it is easy to forget to do these repetitions, so make sure that you have a study schedule that reminds you to go back to the information, and turn it into a habit! Distributed practice may feel difficult and you will forget things in between the study sessions, but this is a good thing. This forces you to retrieve the information from memory and strengthens your memory.
Does it work, and why?
Yes, this strategy is highly useful! One study looked at groups of students reviewing the same material twice, comparing three groups: the two instances immediately after one another, the two instances one day apart and the two instances 30 days apart. The students in the 30-day delay condition performed best. In another (extremely large) study, students recalled more after spaced study (scoring 47 percent overall) than after massed study (37 percent). This means an increase in retention by 10%, not by investing more time but simply spreading the time.
To remember something for one week, learning episodes should be 12 to 24 hours apart; to remember something for five years, they should be spaced six to 12 months apart. Although it may not seem like it, you actually do retain information even during these long intervals, and you quickly relearn what you have forgotten. Long delays between study periods are ideal to retain fundamental concepts that form the basis for advanced knowledge.
Many theories of distributed-practice effects have been proposed and tested. For specific information on these theories, the extensive review by Dunlosky (Improving students’ learning with effective learning techniques: Promising directions from cognitive and educational psychology; 2013) can be consulted. Given the relatively large magnitude of distributed-practice effects, it is plausible that multiple mechanisms may contribute to them; hence, particular theories often invoke different combinations of mechanisms to explain the effects.