Forgetting curve describes the decline in the probability of recall over time.
Examples of curves collected with SuperMemo:
- Power curve for the first review of heterogeneous knowledge
- Exponential curve for the second review of homogeneous knowledge
- Cumulative normalized curve for different levels of stability and memory complexity (over 214,000 data points taken from over 380,000 repetitions)
- 3D curve for various levels of stability (retrievability log axis reversed in reference to time)
Figure: The first forgetting curve for newly learned knowledge. Power approximation is used due to the heterogeneity of the learning material introduced in the learning process that results in superposition of exponential forgetting with different decay constants. On a semi-log graph, the power regression curve is logarithmic (in yellow), and appearing almost straight. The curve shows that in the presented case recall drops merely to 58% in four years, which can be explained by a high reuse of memorized knowledge in real life. The first optimum review interval for retrievability of 90% is 3.96 days. The forgetting curve can be described with the formula R=0.9907*power(interval,-0.07), where 0.9907 is the recall after one day, while -0.07 is the decay constant. In this is case, the formula yields 90% recall after 4 days. 80,399 repetition cases were used to plot the presented graph. Steeper drop in recall will occur if the material contains a higher proportion of difficult knowledge (esp. poorly formulated knowledge), or in new students with lesser mnemonic skills.