In an article for The Telegraph, Grand Master of Memory, Ed Cooke, makes the following observations about the spacing effect:
The reason that forgetting is so rife is that memories fade in time – and only those memories that get repeated are strengthened for the long term. This is usually a good thing, since repetition correlates with importance. Meaningful or important things tend to happen again; random things tend not to. By forgetting what doesn’t repeat, we sort the wheat from the chaff.
The problem with most education lies in a distinction between two ways to go about repetition: spaced and massed. Massed repetition (“cramming”) is when you repeat something over and over in a short period of time. Spaced repetition is where the repetitions are spaced out.
To understand which is better, think of memories as plants in the garden of your memory. Think of repetition as watering. Massed repetition is like watering a plant over and over all at once, and then failing to do so for months. Spaced repetition is like watering the plant once a week for a period of months. The same amount of total watering leads to two very different plants at the end of the story, only one of which is dead. So it is with memory.
A century of scientific research has shown that the very best way to space repetition of material in the service of efficient, long-term learning is in fact to water memories just before they’re about to shrivel, and with gaps that increase with time. Optimally, we want to revisit a new memory roughly after a minute, five minutes, an hour, a day, a week, a month, three months, a year: always catching the memory just before it expires.
Algorithms are the best way to handle this scheduling, but there are two simple things that can be done to worked into an exam revision plan.
First, review what you learn continuously as you learn. After each page of a text book, look back over the main points. After each chapter, review all of its contents. This obviously sounds really boring. But in reality it can be pleasurable, and you’ll learn far faster.
And during revision blasts, look over what you do each day at the end, and each week too. The positive results are staggering: rather than constantly resuscitating dead memories, or over-watering them pointlessly, you can reduce the net amount of time spent by as much as a factor of three.