Forgetting index FAQ
Role of the forgetting index
In SuperMemo, it is possible to control the speed of learning and retention. One of the tools in that optimization is the forgetting index. In incremental reading, natural speed-retention balance is found by the degree of overload and the use of the priority queue. In other words, the role of the forgetting index keeps diminishing. Leaving the default forgetting index of 10% is now a recommended course of action.
Here are some archive FAQs that explain how the forgetting index works
FAQs
Maximum speed of learning is not what you really need!
From: Tomasz Szynalski
Country: Poland
Sent: Oct 18, 1998
Question
What value of the forgetting index ensures the optimum ratio of (retention)/(time spent per day)?
Answer
Paradoxically, the highest speed of learning can be accomplished ... without SuperMemo! In our daily life we pick up lots of facts that stay in our memory for long with few repetitions in lifetime! The problem is that these are usually not exactly the facts or rules that are critical to our goals. In other words, not the speed of acquiring new items counts but the speed of acquiring new items bearing a given content.
It is difficult to determine exactly what forgetting index brings the highest acquisition rate. Simulation experiments have consistently pointed to the value of 25-30%. You can even plot speed-vs.-forgetting graph using your own actual learning material using Toolkit : Statistics : Simulation. You will probably also arrive at similar results.
SuperMemo disallows of the forgetting index above 20%. This comes from the fact that you should aim at achieving high speed of learning combined with high retention of the learned material. Setting the forgetting index above 20% would be like giving up SuperMemo altogether and coming back to remembering only that what is easy to remember. In highly interlinked material where new knowledge depends on the previously acquired knowledge, high forgetting rate can even be more harmful.
Nevertheless, if you want to maximize the speed of learning with little control over what actually stays in your memory, set the forgetting index to 20%
Effect of the forgetting index on retention
From: Tomasz Szynalski
Country: Poland
Sent: Oct 18, 1998
Question
What retention can I obtain with the forgetting index set to 9%? What if I then change it to 12%?
Answer
For the forgetting index of 9%, the retention should be 95.4% (see Forgetting index in SuperMemo). For 12%, the same figure will be 93.9%. Note that if your material is very difficult, your measured forgetting index may be higher than the requested forgetting. This comes from the fact that SuperMemo imposes some boundary conditions on the increase of intervals. Elements that have been forgotten more than five times should be reformulated with a view to reducing their difficulty or increasing their mnemonic component.
If you initially set the forgetting index to 9% and later on increase it to 12%, you will probably start with retention of 94-95% which will later gradually decrease to 92-93% (after the change)
Resetting the measured forgetting index
From: Peter Cool
Country: The Netherlands
Sent: Nov 6, 1998
Question
I started with SuperMemo 70 days ago (your French plus some words added by me; total 1000 words). In the first weeks I made a lot of mistakes so my measured forgetting index was 20%. Although I make very few mistakes now during repetitions the forgetting index decreases very slowly. Is this normal?
Answer
The measured forgetting index includes the record of all repetitions made since you started learning. That is why it changes at an ever decreasing rate. If your performance is good and you would like to more accurately check your current forgetting index, you might reset the forgetting index measurements with Tools : Statistics : Reset parameters : Forgetting index record. It will not affect the learning process per se
In SuperMemo, intervals must increase
From: Manfred Kremer
Country: Germany
Sent: Sep 7, 1998
Question
I noticed that frequently I get optimum interval in the Element data window shorter than the last interval. Is it a bug in SuperMemo?
Answer
No. If your forgetting index is very low, e.g. 3%, SuperMemo will often conclude that you will stand 97% chance of remembering a given item only if your next interval is shorter than the presently used one. In such cases, it will not accept the new value and the new interval will be at least 5% longer than the previous one. Please note that the forgetting index equal 3% should only be used for selected high-priority items. Keeping the forgetting index at this level throughout the collection will make repetitions annoying frequent and ineffective
Intervals are derived from the forgetting curve
From: David Mckenzie
Country: New Zealand
Sent: Apr 8, 1998
Question
Why does not the first repetition after forgetting occur the next day after the unsuccessful repetition (this is advised by Tony Buzan and others)?
Answer
In SuperMemo, the length of the first interval is computed from the forgetting curve plotted in the course of repetitions. This is to make sure that a defined proportion of items is remembered (usually 80-97%). This proportion is determined by the forgetting index. Depending on the forgetting index, the length of the first interval may range from 1 to 20 days, and is not set arbitrarily. It is computed from the record of repetitions and determined by the desired forgetting index (requested forgetting index is the proportion of items that are not remembered at repetitions). While Buzan's recommendation is valid in many cases, you should not forget that SuperMemo computes intervals with a high degree of accuracy. Accurate intervals cannot easily be predicted without a computer
Keep your collections in one piece
From: David Mckenzie
Country: New Zealand
Sent: Apr 8, 1998
Question
Is there any point in keeping collections separate?
Answer
No. Once you master concepts, templates, and subset operations, there is no point. You gain global search, global registries, global repetitions, global optimization, etc. This would not be advisable in SuperMemo 7 as item difficulty measure (E-factor) was dependent on the average difficulty of items in the collection. Presently, the item difficulty measure is absolute and does not depend on the context in which an item is placed. Only the length of the first interval will significantly be affected by the average difficulty of items in the collection. However, this shall not bear dramatically on the speed of learning. Especially that variable forgetting index for individual items makes it possible to set different first intervals for branches of the knowledge tree
Keep an eye on leeches for the low measured forgetting index
From: Noel Clary
Country: USA
Sent: Aug 17, 1998
Question
I have created my own collection on plumbing and air conditioning. My forgetting index is quite high. Are there any tools in SuperMemo which could help me remedy this situation?
Answer
You might want to use View : Other : Leeches and locate the elements that cause most problems in learning. You must then go into your own mind to answer the question why these elements are hard to recall. Usually, these are too complex, too long, too boring or too similar to other elements in the same collection. You can also send 3-4 most difficult items to us for review and offer some suggestions. Read more: Leeches and 20 Rules of Formulating Knowledge
Low forgetting index can also produce long intervals
From: Matt Cassidy
Country: New Zealand
Sent: Sep 11, 1997
Question
Is it possible that with forgetting index equal to 3% I get the first interval equal to 6 days?
Answer
Yes. Especially if the material you work with is relatively easy. You should also remember about the random dispersion of intervals. In isolated cases, dispersion might produce intervals substantially longer (or shorter) than the optimum interval. For more read about SuperMemo Algorithm
Tony Buzan did not contradict SuperMemo
Question:
Tony Buzan claims that 75% of information is lost if not reviewed in 24 hours. Does it not defeat the validity of SuperMemo in which the first interval is often longer than a week?
Answer
No. Buzan's claim may refer to textbook knowledge or complex knowledge structures (e.g. large mind maps). However, it does not seem accurate in reference to simple well-structured material in the light of results obtained with SuperMemo. In SuperMemo, if the student chooses the retention of 95%, the typical value of the first interval falls in the range 2-5 days depending on the student and the difficulty of the learned material. For retention of 25%, the same interval might be as long as one month, though it cannot be verified experimentally with SuperMemo which limits the range of the forgetting index from 3-20%, which implies the overall retention in the range of 89-99%. For more see: Theoretical background of SuperMemo
Users of SuperMemo 2 are annoyed with long intervals
Question
I used SuperMemo 2 shareware, and was accustomed to repeating forgotten items on the next day. It is very irritating that in new SuperMemo, I do not have this possibility
Answer
SuperMemo will schedule forgotten items in intervals that are determined by the forgetting index. The greatest increase in the speed of learning in newer versions of SuperMemo as compared with SuperMemo 2 resulted from substantially increasing the length of the first interval. The student may be left with the feeling that he is likely to forget the item again if it is not repeated on the next day. Statistically, however, he will forget no more than the proportion defined by the forgetting index. By reducing the forgetting index to less than 5%, the length of the first interval is likely to drop to 1-2 days in most cases. Moreover, if you are particular about repeating a given item on the next day, you can press Ctrl+Shift+R to manually modify item's first interval
You can easily change the forgetting index from the keyboard
From: J.P.
Country: Czech Republic
Sent: Sat, Jun 16, 2001 16:59
Question
I find that changing the forgetting index of an element is pretty time-consuming without any keyboard shortcut
Answer
You can use use Ctrl+Shift+P to open the Element parameters dialog box and Alt+F to get straight to the Forgetting index field. You can also change it via Element priority dialog box (Alt+P).
Retention statistic assumes regular repetitions and well-formulate knowledge
From: dansujp
Sent: Sun, Sep 16, 2001 3:07 PM
Question
When I returned from vacation, I expected the retention to be something like 80% because I have not made any repetitions for two weeks. But it was exactly the same as before I left
Answer
The Retention statistic is derived directly from the measured forgetting index on the assumption of a negatively exponential forgetting curve. This curve is only representative of well-structured learning material. In addition, the forgetting index measurements are averaged over all recorded cases. A break in repetitions will invalidate the statistic. Resuming repetitions is not a guarantee of accuracy as a large number of earlier repetitions will result in overestimating the retention on a small-sample measurement. The only valid estimation of retention after a break in learning is the one that follows resetting the past forgetting index record (Tools : Statistics : Reset parameters : Forgetting index record). This will result in gathering new data that will approach the true retention for the sample tested with accuracy proportional to the number of repetitions made
Use the default forgetting index to globally change the forgetting rate
From: Jiri P.
Country: Czech Republic
Sent: Thursday, January 10, 2002 4:28 PM
Question
What is the purpose of the default forgetting index available in Options?
Answer
Tools : Options : Learning : Forgetting Index (default forgetting index) will be used in items that have their requested forgetting index set to Default (internally represented as zero). All items with the default setting will use the default forgetting index currently selected in Options. You can set the default forgetting index with contents and browser operations (Forgetting index : Default forgetting index)
Topics also use a forgetting index
From: Terrence T.
Sent: Feb 14, 2004
Question
What is the meaning of forgetting index in topics? Should not this field be disabled?
Answer
The Forgetting index field in topics has no impact on the way they are handled in the learning process. However, all cloze deletions generated from that topic will inherit its forgetting index. Also, all extracts will propagate the forgetting index to all its descendants. This way, you can reduce the forgetting index on very important articles to ensure better retention of the derived material. Lower forgetting index will also be used to protect your material from postpones when there is a heavy learning overload
A drop in the measured forgetting index is highly undesirable
From: mahabharatta
Sent: Wednesday, March 31, 2004 6:28 PM
Question
My measured forgetting index went up to 90%. I introduced a HUGE load of material over a short time period around 4 months ago. There was a 4-day burden of 900. In the last 3 months I've been introducing new materials sporadically, never more than 100 elements in one day, 300 in a week. During this period the measured forgetting index has been dropping. Today it is 29.9%. Is this normal?
Answer
Forgetting index of 90% tells you, you have introduced too much material at once. The time needed to recover will depend on the degree of overload and the complexity of the material. With forgetting index above 30%, your learning efficiency will not be much better from that achieved without SuperMemo. However, in the long run, if you make order in the learning process, this hectic period can still bring fruit.