Why is nothing going in?
The brain is a muscle: it can learn either by immediate shock (e.g. being burned by a hot stove) or by repeated experience. Unfortunately, this becomes less true for more complex tasks, so it isn’t as simple as reading and re-reading that biography, case note, or list of quotes. The theory of deficient processing might explain the inefficiency of this type of cramming: being presented with the same information multiples times successively reduces attention, due to redundancy not being sufficiently stimulating for information to be memorised.
What should I do instead?
Spaced repetition is certainly better than cramming: “literally hundreds of experiments by cognitive psychologists have demonstrated the advantage of spaced/distributed over massed practice”.
Ali Abdaal, revision-youtuber, doctor, and proponent of a scientific approach to spaced repetition gives his opinion on why leaving a significant gap between two study sessions addressing the same topic is so effective: “the idea is to allow your brain to forget some of the information to ensure that the active recall process is mentally taxing”.
“The optimally efficient gap between study sessions is not some absolute quantity that can be recommended [...] if you want to know the optimal distribution of your study time, you need to decide how long you wish to remember something”. Longer gaps enable deeper learning, whereas knowledge accumulated through shorter intervals will fade away faster. An important rule is that “as the gap increases, accuracy increases steeply and then declines much more gradually”. In practical terms, this means that it’s still better to go through your notes and flashcards once a month rather than risking reading them every three days in the run up for exam, although the former does not exclude the latter. Your revision timetable shouldn’t just take shape in the month before exams, but should be a diary you hold throughout the year: reminding yourself to work on a particular topic the day after you’ve first been exposed to it, then perhaps two weeks later, and then once a month until you no longer need it.
Enablers of the method
Remember, this method is not to be used in isolation: “spaced retrieval practice (with feedback) leads to better retention than spaced rereading” and some theories “emphasize the study/learning context.
In clinical studies exploring the effects of spaced repetition, undertaken by researchers ranging from the University or York to Massachusetts Institute of Technology, students who were given corrective feedback when they got an answer wrong - and were presented with that nugget of information more frequently than information they had correctly recalled - were more likely to learn quicker and more accurately. An easy way to implement corrective feedback into your revision is through flashcards for example, and to further implement the studies’ conclusions, I recommend keeping tabs on how well you have performed in the retention of chunks of information. This could take the form of listing or spreadsheeting the content you need to know, then marking yourself on how well you remember each one, and finally, frequently revisiting lower marked topics!
Although clinical psychology struggles to isolate exactly which mechanisms account for which proportion of the effectiveness of spaced repetition, there is no doubt that a changing environment is beneficial to learning. Glenberg asserts that with changing environments, “the context that gets encoded in memory with each presentation of an item is likely to be more variable [...] the variable contexts that are stored in memory then serve as more effective cues for subsequent retrieval of the item”. He suggests this may be due to the subjective organization in which events are embedded.
In layman’s terms, this means that by changing your study environment, your recall of a concept is not dependent on you being in the same environment as when you encoded it. Paradoxically, you are forced to concentrate on the content of what you are recalling, rather than the format or context of the recall. Once again, implementation is as simple as making sure you don’t stick to revising at the same desk year-round: if you can’t access a library, try the kitchen!
On a final note, as Kang argues in his research paper for SAGE, using spaced repetition certainly promotes efficient and effective learning, but it is important to remember that memorizing instructional content to reproduce verbatim from memory is not the ultimate goal of education. Although your tutors and examiners, much as Kang, will agree that the ability to quickly access relevant information from memory is a prerequisite for “higher order learning and reasoning”, it is only a conductor to the more complex learning, problem-solving, and creative approach you will need to take. The point being, it's better to understand something in your own words than knowing the teaching off by heart.