In the late 19th century, the German psychologist, Hermann Ebbinghaus tested his memory over various periods of time. Once he’d gathered all the data from his spaced learning studies, he plotted it on a graph that looked a little something like this:
The formula with which the Ebbinghaus curve of forgetting is calculated is the following: R = exp(-t/S), with R symbolizing memory retention, t symbolizing time, and S the relative strength of the memory.