Forgetting index in SuperMemo

Choosing the speed of learning

Forgetting index is the proportion of elements that are not remembered during repetitions. It is usually expressed as a percentage. For example, if you choose the forgetting index to be 10%, SuperMemo will try to make sure that you remember 90% of elements at repetition time.

The higher the forgetting index, the faster you learn, but the less you remember!

You can choose the default forgetting index with Toolkit : Options : Learning : Forgetting index (default).

Optimum value of the forgetting index

SuperMemo makes it possible to choose a forgetting index in the range from 3% to 20%. Very often users of SuperMemo ask why there is a limitation on this range. Many of you would be tempted to set the forgetting index to 1% or even 0%. This would only result in unnecessary waste of time. A forgetting index of 0.00% would mean that the intervals between repetitions should equal 0! If you decided to choose a forgetting index of 1%, the repetitions would be so frequent that you would probably become discouraged before ever discovering the power of SuperMemo. In addition, the spacing effect would make your memories very weak. This would shorten intervals further and add more work and frustration.

If you do not have much experience with SuperMemo, you should set the forgetting index to 10%. This value is important for psychological reasons as well. If the forgetting index is too high, your repetitions will be stressful due to constant problems with recall. Your material will seem difficult to remember. This can be quite frustrating. On the other hand, if the forgetting index is too low, your repetitions will be annoyingly frequent. You will experience a sense of wasting your time on needless repetition.

If you feel you remember too little, reduce the forgetting index by one percentage point. If you feel you repeat too often, increase the forgetting index. In most cases, the value of 8%-13% will work best.

Forgetting indexes used in SuperMemo

It is easy to confuse some terminology related to the forgetting index. Here is a short glossary:

• requested forgetting index - forgetting index that you want to achieve in learning. You ask SuperMemo to plan repetitions in such a way that your measured forgetting index reaches a given value. The requested forgetting index may be set for individual items (see: individual forgetting index) or for many items at once. To change the forgetting index for many elements, you can use the default forgetting index or branch operations in the Contents window (Contents : Process branch> : Forgetting index)
• measured forgetting index - forgetting index as it is actually measured at repetitions. It is displayed in the Statistics window as Measured FI. Measured forgetting index is usually higher than the requested forgetting index, esp. if the forgetting index is less than 5%. This comes from the fact that SuperMemo imposes some limitations on the length of intervals in order to prevent excessively frequent repetitions. Secondly, if you delay repetitions, skip repetitions on a given day, or use Postpone, you step away from the optimum learning process and inevitably increase the measured forgetting index. Finally, in an overloaded learning process, the measured forgetting index of low-priority items is higher by design (see: Tools : Statistics : Analysis : Graphs : Forgetting Index vs. Priority). You can restart the measurements of the forgetting index with Tools : Statistics : Reset parameters : Forgetting index record
• default forgetting index - requested forgetting index set for all items with the default individual forgetting index by means of Tools : Options : Learning : Forgetting index (default)
• individual forgetting index - requested forgetting index set for individual items. It can be seen in the Element data window. It can be set with Element parameters dialog box (Ctrl+Shift+P) or Element Priority dialog box (Alt+P). You can also set forgetting indexes for an entire branch in the Contents window with Process branch> : Forgetting index
• expected forgetting index - expected probability of failing to recall a given element if the repetition takes place at a given moment. Used internally by SuperMemo Algorithm. In Algorithm SM-18, retrievability is used instead (R=1-expFI)
• estimated forgetting index - value of the forgetting index derived from the scored grade on the basis of the correlation graph showing the relationship between the expected forgetting index and the average grade obtained at repetitions (graph G-FI graph). Used internally by Algorithm SM-15. This concept is no longer applicable to Algorithm SM-18. However, SuperMemo 18 still uses grade-retrievability correlations in some contexts

Retention vs. forgetting index

If you set your forgetting index to 10%, you will remember 90% of the material at repetitions. This does not imply that your knowledge retention will be 90% only. Your average retention will be nearly 95%! This comes from the fact that 90% refers to the retention at repetitions, while the initial retention right after the repetition is theoretically 100%. During the inter-repetition interval, retention is decreasing from 100% to 90%. On average you roughly remember 95% of the material. The exact formula linking the forgetting index with the retention is as follows (source):

retention = -(forgetting index)/ln(1-(forgetting index))

Forgetting index	Retention
3%	98.49%
5%	97.47%
10%	94.91%
15%	92.29%
20%	89.62%

The reason that the retention is not equal to 1-0.5*(forgetting index) is that forgetting is approximately exponential in nature. Immediately after the repetition, forgetting proceeds at the fastest rate.

Do you know that...?

• you can sort elements by the forgetting index by clicking the FI column in the browser (browsers are available from the View menu)
• you can search for elements with a given requested forgetting index by means of View : Filter
• you can see how SuperMemo plots the forgetting curve graphs and derives the length of the interval from the forgetting index. Try Toolkit : Statistics : Analysis : Forgetting Curves
• you can see correlations between the forgetting index and other learning parameters that have been measured in a group of 32 students
• the concept of the forgetting index was introduced only in SuperMemo 6 for DOS in 1991. Earlier versions of SuperMemo tried to accomplish 10%. This value could not be changed. SuperMemo 1.0 and SuperMemo 2.0 used a hit-or-miss algorithm that was still pretty good at reaching 85% retention
• individual forgetting indexes are kept in the file [INFO]\ElementInfo.dat and the default forgetting index is kept in the file <collection name>.kno (for details see: SuperMemo files)

Changing the forgetting index

1. You can change the default forgetting index in your collection with Toolkit : Options : Learning : Forgetting index (default)
2. You can also change the forgetting index of elements with the Element parameters dialog box (Ctrl+Shift+P) as well as with the Element Priority dialog box (Alt+P)

FAQ

See also: Forgetting index FAQ