If we do not review and revisit the material, it leaves our memory very quickly. In the first 24 hours alone we forget about 30% of what we learned.
Below are examples of two students—one student who reviewed their course materials throughout the semester, and one student who didn't.
Student A: Reviewed Notes
The student in this example forgets 30% of a lesson in 24 hours. However, she works with the material within 24 hours. Maybe she re-copies her notes, outlines the chapter she read, or works the problem sets.
Over the next week she gradually loses about 20% of the material again, but she revisits it again in her weekly review. Maybe this time she puts her notes into study guide format or creates a practice test with a study buddy. Through this she is able to get back up to the 100% mastery she had a week prior—in about a third of her original time investment.
Then about four weeks later, she does some more reviewing because she has a test coming up. Over that month she only loses about the same 20% she did in that first week, because she has done the daily and weekly review, moving information into long term memory. Now she needs to spend very little time brushing up, and she’s ready for the test.
Student B: Did No Review
Meanwhile, Student B classmate also mastered the material to start off, but did no daily, weekly, or pre-exam review. This student lost the same 30% in the first 24 hours, and this continued to degrade another 5%–15% over the month, leaving him going into the exam with only about half of the information still in his memory.