How to use it
The grey line is one reading with no review. The orange line is the same reading with a review on each day that you select.
Select a day to add a review, and select it again to remove it. “Axo’s schedule” puts the reviews on days 1, 3, 7 and 14. Move over the chart to read both lines on any day.
Change the material from Normal to Hard and both lines fall faster. Change it to Easy and they fall more slowly.
What you will see
One review does more on day 1 than on day 21, because on day 21 most of the memory is already gone. Reviews with gaps that get longer keep the line higher than the same number of reviews close together at the start.
Try it: select days 1, 2 and 3, and read day 30. Then select days 1, 7 and 21.
The model
Memory is drawn as R = e^(−t/S). R is the part you still remember, t is the time in days since the last review, and S is the stability of the memory. A larger S gives a flatter curve.
Each review puts R back to 100 percent and multiplies S by 2.3. Normal material starts with S at 2.4 days, Easy at 3.6 and Hard at 1.6.
These values are chosen to give the shape that research describes. They are not measured from you. A real memory depends on the material, on how well you learned it, and on you.
Where the idea comes from
Hermann Ebbinghaus measured the first forgetting curve on himself in 1885. A team in Amsterdam repeated his experiment in 2015 and got a curve of the same shape. The article on the forgetting curve has his numbers and both sources.