Which interval do you want to use

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Executive summary

When SuperMemo asks you Which interval do you want to use? and you are not sure, press Enter and choose the default suggested by SuperMemo 17. Use other options if you are anxious about astronomically long intervals.

Which interval do you want to use?

SuperMemo 17 uses a new algorithm. At the same time, it runs a parallel computations of the older Algorithm SM-15. In certain circumstances, the intervals proposed by the two algorithms may differ. In addition, SuperMemo imposes constraints on interval change. For example, in the past, SuperMemo always used to make sure intervals do not decrease. In new SuperMemo, DSR model suggests that in cramming, memories can weaken and intervals may decrease. You may wish to prevent interval decrease.

Most of interval differences have little to do with how memory works. They are more about user strategies. For example, a user may wish to keep intervals short even if SuperMemo knows the item is remembered well.

To assist the user in making his or her choices, SuperMemo may display a question Which interval do you want to use?.

If you want to suppress this dialog and default to SuperMemo 17 algorithm use Tools : Options : Learning : Algorithm SM-17 Alerts. For example, put a large Interval value (e.g. 3000).

Choices (in SuperMemo 18)

Here are the interval options:

  • SuperMemo 18 suggests: optimum interval proposed by Algorithm SM-18 in agreement with the current status of memory for the considered item. Interval decrease may be prevented in shorter interval ranges. See: Case against cramming
  • SuperMemo 15 would use: interval that SuperMemo 15/16 would have used. It might be very short if the user has previously manually shortened the interval, even if the item is remembered very well
  • DSR model suggests: optimum interval proposed by the DSR model of memory in agreement with the current status of memory for the considered item. This interval may be shorter than the used interval, e.g. in cramming
  • Minimum interval: minimum interval determined by the previously set interval incremented by one day (old interval+1). For example, if the previously set interval for the item was 3 years, minimum interval will be 3 years and 1 day
  • Minimum non-regressive interval: minimum interval determined by the used interval incremented by one day (used interval+1). For example, if the item has previously been reviewed a week ago, this interval will be 7+1=8 days
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Choices (in SuperMemo 17)

Here are the interval options:

  • SuperMemo 17 interval: optimum interval proposed by Algorithm SM-17 in agreement with the current status of memory for the considered item (interval decrease may be prevented in shorter interval ranges). Forgetting index is ignored
  • SuperMemo 16 interval: interval that SuperMemo 16 would have used. It might be very short if the user has previously manually shortened the interval, even if the item is remembered very well
  • DSR model interval: optimum interval proposed by the DSR model of memory in agreement with the current status of memory for the considered item (this interval may be shorter than the used interval, e.g. in cramming)
  • Minimum interval: minimum interval determined by the previously set interval incremented by one day (old interval+1). For example, if the previously set interval for the item was 3 years, minimum interval will be 3 years and 1 day
  • Minimum non-regressive interval: minimum interval determined by the used interval incremented by one day (used interval+1). For example, if the item has previously been reviewed a week ago, this interval will be 7+1=8 days

Notes

  • Minimum non-regressive interval will often be longer than minimum interval. For example, if SuperMemo planned a review in 7 days, but the user came late and executed the review in 14 days, the minimum interval will be 8 days, but minimum non-regressive interval will be 15 days
  • In SuperMemo 16, manual intervention leaves a permanent mark on the learning process. For example, if the user cuts the interval from 3 years to 3 months, the item will be subsequently reviewed as if its optimum interval was 3 months indeed. This makes it possible to intervene once, and never worry about the item again. In SuperMemo 17, cutting the interval from 3 years to 3 months will work only for that one interval. In the next review, SuperMemo 17 will always compute the status of memory and will likely propose a long interval again (e.g. 4 years). This is the main source of differences between the two algorithms
  • SuperMemo 17 may conclude that the next optimum interval is actually shorter than the current interval. This may be the case for very difficult, often badly formulated items. This can also happen in cramming. Older SuperMemos would always employ non-regressive intervals. In the past, the intervals would never get shorter. Using the interval dialog you can choose between shorter interval (DSR Model) or anti-cramming interval (SuperMemo 17). For example, the dialog may propose very long intervals across the board except for the optimum interval determined by the DSR model. You can choose this shorter interval, however, you should also have a look at the item. Perhaps it should be simplified.
  • minimum intervals are not too useful in regular work, however, may come handy when resuming work after a longer break, or for beginners (see: Choice about what interval to use)

Suppressing the interval question

If you want to always rely on the algorithm, ensure high speed of learning and efficient memory stability calculations, you can suppress the above dialog. This will take away some of your control over how SuperMemo behaves, but it will also speed up the process of reducing your repetition workload.

To suppress the dialog use Tools : Options : Learning : Algorithm SM-17 and provide high values for Difference and/or Interval. For example, by setting Interval=3650, you will make sure the interval dialog will bother you only in cases where SuperMemo 17 proposes intervals longer than 10 years (3650 days).

Why ever use minimum interval?

Why would I ever want to use the minimum interval, which I understand as "no progress"? The whole point of SuperMemo is to use optimum intervals, isn't it?

When minimum is not minimum, and optimum is not optimum

SuperMemo 17 is true to your memory performance. On occasion, it can tell you that you should shorten the next interval! This may happen if the question is very difficult, and you are not really sure of the answer. You may then happen to provide the right answer. This is even more likely if you delay the review by a large margin. In lay language, it is a situation where you "answer correctly even if you should rather fail" (i.e. statistically, the correct answer is unlikely).

This is a real life example:

Which interval do you want to use?

SuperMemo 17 suggests: 10yr 2mth 30 days
SuperMemo 16 would use: 13yr 1mth 29 days
DSR model suggests: 3yr 1mth 2 days
Minimum interval: 9yr 1mth 14 days
Minimum non-regressive interval: 9yr 1mth 14 days
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You will notice that minimum interval is actually longer than the optimum interval suggested by the DSR model. At the same time, SuperMemo 16 rigidly enforces its two constraints: (1) intervals cannot get shorter, and (2) U-factor cannot drop before a set minimum value.

Your decision here will depend on your learning criteria and the importance of the item. Using DSR model interval will satisfy your forgetting index goals, but will slow down learning. Using SuperMemo 16 will satisfy your speed of learning goals, which is ok as long as you ruthlessly eliminate leech items (i.e. items that are hard to remember due to bad formulation). Choosing one of the minimum intervals falls in between in this particular case, and would be a statement to this effect: "I do not perform well with this item, but I refuse to make any step back".

See also