In June 1991, a journalist of Computer World, Andrzej Horodenski, noticed that for lazy users of SuperMemo an option called Mercy would be extremely useful. It would allow users to reschedule outstanding repetitions after a vacation period. Just one month later, SuperMemo World released a new version of SuperMemo 6 for DOS that included the suggested option.
The first Mercy algorithm was based on a solid theoretical ground. The item sorting criterion was to minimize the drop of retention as a result of using Mercy. However, the choice of the sorting criterion was not very fortunate. It is easy to notice that an increase in intervals of short-interval items is more detrimental to retention than the same increase for long-interval items. Consequently, users abusing Mercy would pile up lots of hard-to-remember items that would recur again and again contributing to overall discouragement of the overwhelmed mind. It is difficult to estimate how many people got hooked on Mercy and dropped from among the users of SuperMemo.
A second option was then added to SuperMemo 6. It was called Wipe and it was supposed to remove from the learning process all short-interval items with a high degree of difficulty (expressed then by E-Factors). However, Wipe might have done more damage to SuperMemo than ill-conceived Mercy. Users would often pile up items with Mercy and then get them out of the learning process with Wipe. Soon they could see that no real progress in learning was taking place. As a result, they would drop out again with detriment to overall popularity of SuperMemo.
In 1994, a new Mercy algorithm was designed. The new sorting criterion: minimize the damage to the long-term learning process. The algorithm appeared extremely intricate but has changed Mercy beyond recognition. Indeed, it could be seen very soon that the option Wipe became entirely superfluous. New Mercy would be as abused by the users as the old one, but it would result in less damage and less discouragement. Mercy survived and was not removed from subsequent versions of SuperMemo. After all, we believe that we should give people a chance to decide what they need, even if they can occasionally harm themselves.
It became clear only much later that the second Mercy had a hidden snag. If abused frequently, it was able to repeatedly lengthen the first interval of newly memorized items (after all, they are supposed to be less important for the long-term learning process). This problem was compounded by the fact that all SuperMemo algorithms implemented in the years 1989-1996 were highly sensitive to delaying repetitions, esp. at the early stages of learning. Consequently, items that were dramatically postponed by Mercy and that would score well in repetitions would have reached disproportionately long intervals.
These problems have been partly solved in December 1996 by implementing the following features in SuperMemo:
1. removing sensitivity to repetition delay of the Algorithm SM-8 by indexing optimization matrices with repetition categories rather than repetition numbers (for example, a item repeated twice would be treated as repeated 2.4 times in case of repetition delay).
2. implementing new multicriterial heuristic Mercy algorithm that combines its earlier emphasis on minimizing the damage to the long-term learning process with adding extra attention to items that have recently been introduced to the knowledge system.
Finally, in December 1997, Mercy has been enhanced with the option Criteria. This option makes it possible for the user to choose his or her own Mercy criteria. Hopefully, this is the end of the 5-year-long process of coming to understand of what really people expect from Mercy. The following sorting criteria can be balanced by the user: item importance (as indicated by the ordinal and the forgetting index), repetition lateness, investment (in the item), easiness (of the item) and recency of introducing the item to the learning process. The last one is particularly important in knowledge system when recently introduce knowledge is of paramount importance.
In SuperMemo 8, Mercy is available from Tools : Mercy.