SuperMemo Algorithm
From SuperMemo Help
Contents

Introduction
SuperMemo had a fertile impact on the research of modeling longterm memory. One of the most interesting fruits of that research is the two component model of longterm memory. Over the last two decades, it has been hinted on many occasions that the model may provide a sound theoretical basis for a better and more universal approach to spaced repetition. Algorithm SM17 (AlgSM17 or just SM17 in short) is an attempt to convert the theory into a practical application. Due to its universal nature, the algorithm should, in the future, find its way to all SuperMemo products.
Important! At the moment of writing (April 2016), SuperMemo 17 does not use incremental adjustments to optimization matrices in Algorithm SM17. This is why you should execute Tools : Memory : 4D Graphs : Stability : Compute from time to time to adjust the algorithm to newly available data (use F7 and click Compute). In the future, the adjustments will be made at each repetition.
Two component model
The two component model of longterm memory underlies Algorithm SM17. The model asserts that two variables are sufficient to describe the status of unitary memory in a healthy human brain:
 stability  this variable determines how long a memory can last if undisturbed and if not retrieved
 retrievability  this variable determines the probability of retrieving a memory at any given time since the last review/recall
In the literature on memory, an illdefined term of memory strength will often stand for either stability (S) or retrievability (R). This results in a great deal of confusion that slows down the progress of memory research. In 1995 (and earlier), we have proven theoretically that the two variables: R and S are sufficient to describe the status of memory in spaced repetition.
Algorithm SM17 is the ultimate practical proof for the correctness of the two component model.
A graph of actual changes in the value of the two components of memory provides a conceptual visualization of the evolving memory status:
Figure: Changes in memory status over time for an exemplary item. The horizontal axis represents time spanning the entire repetition history. The top panel shows retrievability (tenth power, R^10, for easier analysis). Retrievability grid in gray is labelled by R=99%, R=98%, etc. The middle panel displays optimum intervals in navy. Repetition dates are marked by blue vertical lines and labelled in aqua. The end of the optimum interval where R crosses 90% line is marked by red vertical lines (only if intervals are longer than optimum intervals). The bottom panel visualizes stability (presented as ln(S)/ln(days)
for easier analysis). The graph shows that retrievability drops fast (exponentially) after early repetitions when stability is low, however, it only drops from 100% to 94% in long 10 years after the 7th review. All values are derived from an actual repetition history and the DSR model.
Past vs. Future
All SuperMemo algorithms before the year 2015 took one essential assumption that limited their universal applicability in learning: the review of the learning material should occur at optimum timing determined by SuperMemo. Algorithms employed in SuperMemo 11 through 16 included weak heuristics that would allow of repetition at any time without a noticeable detriment to learning. In particular, those heuristics would help overcome the impact of the spacing effect where frequent reviews lead to less impact or even weakening of memories. New research indicates that the applicability of those heuristics was limited to the less extreme departures from the optimum schedule. The new algorithm, in theory, should make it possible to optimize review for any repetition history, and any repetition timing. It should also make it possible to simulate the status of memory produced with different variants of spaced repetition algorithms (e.g. those used by SuperMemo in remote past).
Weaknesses of Algorithm SM15
Here is the list of major weaknesses of Algorithm SM15 as revealed by comparisons with Algorithm SM17:
 lack of the retrievability dimension in mapping the function of optimum intervals
 weak spacing effect heuristic (e.g. performing poorly in highly massed repetitions)
 memoryless difficulty determination method (new AFactor would be derived from the current estimated forgetting index and the OFactor matrix without taking into account past difficulty estimates and repetition history)
 incremental changes to item difficulty would produce nicelooking difficulty distributions that would not truly reflect actual item difficulty
 heterogeneous streams of items passed via the optimization matrices resulted in clusters of "polluted" entries
 relying on the best fit approximation of optimum factors would not reflect variations caused by user strategy, or item heterogeneity
 lack of filtering for the power law in heterogeneous forgetting curves. In particular, the first review forgetting curve is better approximated with a power function, which shows only in collections with substantial first review delay (beyond 100200 days)
 excess reliance on the weak correlation between grades and the estimated forgetting index
 weak first grade vs. AFactor correlation. For a flat correlation only extreme values of AFactor would be used after the first repetition. Such a flat correlation would often develop in wellmanaged old collections
 relying on the measured forgetting index as the main optimization criterion may be unreliable for heterogeneous learning material where difficulty streams are not well separated
 lack of tools for maximizing the speed of learning for varying levels of the forgetting index
 lack of tools for maximizing the increase in memory stability for varying levels of the forgetting index
 first postlapse interval would not differentiate between items whose forgetting was caused by different degrees of delay (rather than overestimated difficulty)
 assuming optimality of the schedule: manual interventions into the length of intervals result in discarding the stability record, i.e. wellremembered items with short intervals are treated in the same way as other items with the same interval length and with the same difficulty.
See also:
Strengths of Algorithm SM17
 extending review optimization to various levels of recall (from cramming to decadelong delays)
 optimum interval is always based on the full history of repetitions
 databased improvements to the algorithm can instantly be applied in the learning process by recomputing optimization matrices at 500,000 reps/min (previously, it would take months and years to collect corrective data)
 best fit difficulty determination
 universal memory formulas make it possible to simulate any imaginable learning outcomes, e.g. computing the efficiency of older algorithm post factum
 universal memory formulas make it possible to simulate review scenarios for individual items, e.g. computing maximum learning speed schedules, or best average retention schedules, or probability of recall at chosen interval, etc.
 separating the first review forgetting curve and the first postlapse review forgetting curves from the forgetting curve matrix for the sake of keeping homogeneous flow of items when computing changes in recall and memory stability
 employing power law for the first review forgetting curve as more accurate for longinterval review. Shorter first interval should be well compensated by faster increase in successive intervals for wellformulated items
 less sensitivity to chaotic oscillation in memory stability caused by inaccurate stability or retrievability estimates. Those oscillations are not visible in older algorithms as stability estimates were not available to juxtapose against the interval used in the learning process
 expressing first postlapse interval as a function of retrievability (in addition to the number of lapses) yields better RecallRetrievability matching later in the learning process
 expressing first postlapse interval as a function of retrievability makes it easier to estimate the difficulty of forgotten items by filtering out lapses caused by delays
 35quantile forgetting curve for the first review spans a decade (rather than just 3 weeks as in older versions of the algorithm)
Weaknesses of Algorithm SM17
 unlike all prior versions of the algorithm, Algorithm SM17 requires a full record of repetition history for all items. This adds an additional burden on developers and licensees
 onetoone mapping between recall and retrievability remains elusive. This is the main validity test for the underlying theory of memory. There is still room for improvement, esp. for difficult items
 older versions of the algorithm would keep 255 most recent repetition outcomes when plotting multidimensional forgetting curves. Algorithm SM17 keeps the full record of repetitions. This may make it more "conservative" in adapting to user's own progress (e.g. in formulating items). However, users of automemorized Advanced English 2014 showed dramatically bad recall predicted by the first forgetting curve in SuperMemo 16. This would be less prominent in Algorithm SM17. Amending the inflexibility of Algorithm SM17 is still in consideration (e.g. by recency weighing)
Theory
Two component memory model
The implications of the two component model of longterm memory are described in this 2005 article:
Due to the fact that reallife application of SuperMemo require tackling learning material of varying difficulty, the third variable involved in the model is item difficulty (D). Some of the implication of item difficulty have also be discussed in the above article. In particular, the impact of composite memories with subcomponents of different memory stability (S). The three component model of memory is referred to as the DSR model.
For the purpose of the new algorithm we have defined the three components of memory as follows:
 Memory Stability (S) is defined as the interrepetition interval that produces average recall probability of 0.9 at review time
 Memory Retrievability (R) is defined as the expected probability of recall at any time on the assumption of negatively exponential forgetting of homogenous learning material with the decay constant determined by memory stability (S)
 Item Difficulty (D) is defined as the maximum possible increase in memory stability (S) at review mapped linearly into 0..1 interval with 0 standing for easiest possible items, and 1 standing for highest difficulty in consideration in SuperMemo (the cut off limit currently stands at stability increase 6x less than the maximum possible)
See also: Modeling memory
Historical note: three variables of memory
The three variables of memory were introduced gradually into SuperMemo algorithms:
 Difficulty (D) was first introduced in SuperMemo 1.0 (1987) in the form of EFactors that reflected difficulty. EFactors would decrease each time items performed poorly in learning
 Stability (S) was first hinted at in SuperMemo 5.0 (1989), which was able to demonstrate the decline in stability increase with longer intervals (decrease in OFactors with the increase in repetition number)
 Retrievability (R) was first accounted for in SuperMemo 11 (2002), which used a heuristic formula to compensate for spacing effect in massed presentation and in delayed review
All three variables can now be computed precisely using repetition histories in Algorithm SM17.
Deviations
To assess how well computer models fit reality, SuperMemo uses its own deviation measures. In particular:
 the deviation between grades and predicted grades derived from retrievability when computing the SInc[] matrix, and
 the deviation between matrices and theoretical symbolic function (in particular, the entries of the SInc[] matrix and the theoretically predicted values of the stability increase function Sinc()).
Grade deviation
SuperMemo converts grades to average retrievability correlated with those grades for a given user (every user has his own system of grading and different levels of average retrievability will correspond with individual grades). The gradederived deviation (gDev) is the difference between gradederived retrievability (Rg) and theoretical retrievability (Rt) derived from stability estimate (Sw). The failsuccess deviation (fDev) is 1R for grades>2 and R for grades<3.
In estimating the deviation of grades from predicted grades, SuperMemo uses the formula:
Dev=BGW*fDev+(1BGW)*abs(gDev)where:
 Dev  prediction deviation
 BGW  binary vs. gradebased weight (currently chosen to be 70%)
 fDev  binary successfailure deviation
 gDev  grade derived retrievability deviation
The deviation for an item repetition history is computed as the square root of the average of squared deviations (Dev) for each repetition.
The deviation for a collection is computed as the square root of the average of squared deviations (Dev) for each repetition in each repetition history of each item.
In least squares comparison, for items or for [[Glossary:Collectioncollections], the last squares deviation is:
LSDev=sqrt(sum(square(Dev))/count)where:
 LSDev  least squares deviation for all grade predictions (Dev) in a subset of repetition histories
 count  number of repetitions in the set
 sqrt  square root
The assumed weighing (BGW) is a heuristic based on experiments and the fact that graderetrievability correlation makes for a weak prediction of grades based on R. The said deviation formula is subject to change.
The deviation used in estimating difficulty is weighed further to increase the impact of more recent repetitions on the current difficulty estimate.
Currently, for large collections, the algorithm can generate stability increase matrix which produces a deviation of 18%. Considering the stochastic nature of forgetting, this number would be close to the lowest possible theoretical value if only failsuccess deviation (fDev) was used. However, gradederived deviation (gDev) reduces the span of the deviation (by making R predictions more precise).
Theoretical minimum to a grade deviation
When using predicted grade deviations in assessing the strength of particular algorithmic solutions, we need to keep in mind that there is a theoretical limit on the minimum value of the deviation. This means that we will never approach the ideal 0% deviation.
For the formulas used above, the limit will depend on weights (e.g. BGW) and particular user's graderetrievability averages.
If only binary successfailure deviation (fDev) was used, and all items were scheduled with a perfect interval at the requested forgetting index of 10%, the minimum deviation would correspond with the retrievability prediction of 0.9. That deviation would then be:
sqrt((RFailure)*(RFailure)*P(failure)+(RSuccess)*(RSuccess)*P(success))
which is:
sqrt((0.90)*(0.90)*0.1+(0.91)*(0.91)*0.9)=30%
In addition to the least square deviation, SuperMemo 17 will provide a few other metrics for assessing the effectiveness of the algorithm. If you want to find your metric in SuperMemo options, please join the discussion: Spaced repetition algorithm metric
Matrix deviation
SuperMemo uses matrix representations to express various functions, and hillclimbing approximations to find best theoretical symbolic matches for those functions for use in various contexts.
To guide the hillclimbing algorithms, a separate deviation measure is used as an optimization criterion. For each individual matrix entry:
Dev=sqr(abs(MatrixValueApproximationValue)/MatrixValue))
The deviation for the entire matrix is the square root of the average of individual deviations (Dev). SuperMemo expresses that deviation in percent.
Approximations
SuperMemo uses hillclimbing algorithms to match hypothetical functions that might model data categories of interest for the algorithm. Those functions can later be used to speed up calculations of Algorithm SM17. Currently, three best fit approximation procedures are used to verify theoretical formulas used in the algorithm. Those approximations determine the following functions:
 stability increase (arguments: D, S, R)
 recall (arguments: D, S, R)
 first postlapse interval (arguments: Lapses, R)
Those approximation procedures may also be used in research that helps validate the algorithm. For example, to prove the exponential nature of forgetting, take additive and multiplicative variants of approximation formulas for Recall[D,S,R] with separate set of parameters defining power and exponential components along the R dimension. Outcome: despite the fact that heterogeneous forgetting curves may appear to follow the power law, Algorithm SM17 is filtering items for difficulty well enough to eliminate the power component entirely from the equation. Note that the forgetting curve describing the first review does not form a part of the Recall[] matrix. This is because a heterogeneous stream of new items might pollute the matrix and initiate the "ripple effect" of distorted estimates for higher stability levels (as in older SuperMemos). That first review forgetting curve follows the power law pretty well and, unlike in older SuperMemos, power regression is used to produce the default startup stability for new items.
Approximation parameters in student assessment
There are several hypothetical correlations between approximation parameters and characteristics of user's learning process and knowledge quality. For example:
 RMax in stability increase may be a good overall indicator of the quality of learning (the higher the RMax, the closer the approach to the optimum characterized by minimum information principle)(RMax is retrievability that maximized the increase in stability)
 S Decay in stability increase may be a good indicated of the quality of knowledge (the worse the formulation, the higher S Decay)(S Decay is the decay constant determining how fast SInc[] decreases in the S dimension)
 Recall Intercept in recall approximation is higher in well formulated collections (Recall Intercept is a measure of the agreement between Recall[] matrix and the theoretical exponential decline in recall with time)
The Algorithm
Intro
Algorithm SM17 computes new intervals using the stability increase function. This function tells SuperMemo how much intervals should increase for all possible combinations of memory status variables: Difficulty, Stability and Retrievability. SuperMemo determines item difficulty by finding which difficulty level provides the best fit to grades scored in repetition history. The first interval is computed in a way that is similar to earlier SuperMemos. The stability increase function is computed using data collected during repetitions.
Stability increase function
Central to Algorithm SM17 is the stability increase function represented by the stability increase matrix (abbreviated to SInc[]). The simple formula at the core of SuperMemo is:
Int[n]=Int[n1]*SInc[D,S,R]
This means that optimum intervals in SuperMemo increase by SInc[] in each repetition (assuming the requested forgetting index is 10%). D, S, R stand for 3 memory variables: difficulty, stability and retrievability respectively.
Figure: 3D graph of SInc[] matrix based on 60,167 repetition cases for items with difficulty=0.5. The increase is dramatic at low stabilities (17fold increase is unheard of in earlier SuperMemos), and peaks at retrievability of 0.85. In some cases, the SInc drops below 1.0, which corresponds with a drop in stability (i.e. memory lability). Those low values of SInc do not depend on retrievability. Those huge variations in SInc[] are the main reason why SuperMemo 17 beats SuperMemo 16 in learning metrics by a wide margin..
Startup stability and startup interval
Before the formula for the increase in the length of intervals can be used, we need to determine the initial stability and the first interval to use in repetitions:
 startup interval (SI) is the common first interval for all new items determined by the forgetting curve for new heterogeneous material
 startup stability (SS) is the estimate of stability after the first review determined by the best match to grades obtained in later repetitions (that estimate may change with the arrival of new data)
 postlapse stability (PLS) is the stability of an item after a repetition that scored a failing grade
The relevant formulas used to determine first intervals in SuperMemo are:
Int[1]=SI (for the first repetition)
Int[1]=PLS (for the first review after a failing grade)
Int[2]=SS*SInc[D,S,R]
Historical note: adding retrievability dimension
The SInc[] matrix is similar to the optimum factor matrix from earlier SuperMemos (OF matrix in SuperMemo 5.0 through SuperMemo 16.0), but it takes different arguments. In the same way as OFmatrix determined the increase of intervals, SInc[] is used in the exactly same way except it takes three arguments: Difficulty, Stability and Retrievabilty. OFmatrix was indexed by AFactors (reflecting Difficulty) and repetition category (roughly related to Stability). However, OF matrix did not include the Retrievability dimension as repetitions were supposed to be executed at optimum intervals determined by the requested forgetting index.
Item difficulty
Difficulty: Definition
Item difficulty in SuperMemo Algorithm SM17 is a number from 0 (easiest) to 1 (hardest). Items are linearly mapped in the 0..1 difficulty range by using their maximum possible memory stability increase. Maximum increase in stability occurs in the first review when the length of the interval corresponds with retrievability that produces the greatest increase in the strength of memory. Items with theoretically highest possible stability increase are assigned difficulty of 0. These are the easiest items in the process. Those items are the target of all forms of learning as they are a perfect reflection of the minimum information principle. As the maximum stability increase for very difficult items may approach 1.0 (or even drop below that value, i.e. produce memory liability!), all items at certain lowest acceptable maximum stability increase are classified as having difficulty 1.0. The exact cut off number is the parameter of the algorithm that is subject to change and further optimization (see Value N in Computing difficulty algorithm below)
Historical note: measuring difficulty
As of Algorithm SM8, SuperMemo used the concept of AFactor to define item difficulty. AFactor was defined as the increase in optimum interval during the first repetition (RepNo=2 in SuperMemo). In most situations, AFactor would fall in the 1.010.0 range. SuperMemo employed the range from 1.2 to 6.9 where 1.2 limit was set for the sake of filtering out the socalled "leech material", i.e. material whose difficulty would disqualify it from efficient use of spaced repetition. AlgSM17 might have used that AFactor definition or strive at more universal concept of maximum increase in memory stability. Those two approaches do not need to differ much as newer data indicates that the maximum stability increase occurs at retrievability (R) that are close to the default levels used in SuperMemo. AlgSM17 normalizes difficulty in the 0 (easiest) to 1 (hardest) range to roughly correspond with AFactors 1.1 to 10.0. The assumption is that items that are harder than 1.0 should be eliminated from the learning process, while items that are easier than 10.00 are either extremely rare or can safely be treated along the rest of 10.0 items without a detriment to total workload.
Computing difficulty
AlgSM17 requires a full history of repetitions to compute item difficulty (see below for a simplified procedure that goes around that requirement for developers who might wish to develop light SuperMemos that might look at smaller data sets for mobile devices, etc.).
For the full range of possible item difficulties, AlgSM17 computes the deviations of predicted grades (based on a given assumed difficulty) from actual grades as recorded in repetition history of a given item. The algorithm chooses the difficulty that produces the least deviation.
An important parameter in computing grade deviation in difficulty estimates is the deviation exponent. Least squares approach is prevalent in statistics, however, different values of the deviation exponent yield slightly different results. The ultimate goal is to set all parameters of optimization so that to produce a closer adherence of predicted outcomes to theoretical models.
Computing difficulty: Algorithm outline
Computing difficulty:
 Set S0 to the default optimum first interval from the first forgetting curve for mixed difficulty material
 For all difficulty quantiles, using Algorithm SM17, compute the deviation of predicted grades based on that difficulty, S0, and the actual repetition history from the grades scored in that repetition history
 Find maximum and minimum quantiles with lowest deviation (very often, several quantiles will produce the exactly same deviation as a real number retrievability is converted to grades, which are integers)
 Assume that the item in question has a difficulty that corresponds with the average of minimum and maximum quantiles that generate the minimum deviation
Note that weighing quantiles by deviations tends to amass most difficulties around 0.5, which shows that this is not a valid approach.
Computing the deviation:
 set S to S0
 for each repetition, using the interval, compute new S using SInc[] matrix indexed by the assumed difficulty, S, and R (corresponding with the interval)
 for each repetition, using S and interval, compute R and convert it to a predicted grade (using R:grade mapping characteristic for the user)
 compare that grade to the actual grade and add the difference to the deviation
Some important considerations in difficulty calculations:
 recent repetitions have a greater weight than older repetitions due to the fact that difficulty is subject to change over time (e.g. due to new learning or minor item reformulation)
 instead of squared deviations, SuperMemo seeks to find the optimum exponent in weighing grade deviations to find the best match to theoretical memory models
 difficulty can be computed using theoretical SInc() function, or actual SInc[] matrix data. The outcomes will differ. In particular, the range of minimum and maximum quantiles with the minimum deviation may be larger when using real/irregular data. The value of both approaches is currently under intense study
At the moment, the most likely solution to be adopted in SuperMemo is to use BestSInc() function that transitions from SInc() towards SInc[] with the gradual inflow of data.
Theoretical SInc() function is the root where the baseline mapping of maximum stability increase to difficulty is established:
SInc:=(5*(1Difficulty)+1)*f(R,S)+1where:
 SInc  stability increase
 Difficulty  difficulty in 0..1 range
 f(R,S)  component of SInc that depends on R and S
This baseline formula currently determines the cutoff value for hardest possible items at SInc that is N times less than the maximum value possible for easiest items (with N=(6*f(R,S)+1)/(f(R,S)+1)).
Computing difficulty: Simplified procedure
We have also developed a simplified solution for computing item difficulties. This might be useful for developers who look for simplicity, thinner and faster code. Simplified procedure is similar to the approach used in older SuperMemos, esp. SuperMemo 8 through SuperMemo 16.
Item difficulty may be made historyless by relying on the fact that difficulty of an item keeps evolving depending on the context of knowledge and through interference. For that reason, the performance in the most recent repetition counts more than performance in prior repetitions.
Simplified difficulty can then be obtained using the formula:
D[n]=sDF(D[n1],Grade,Rr)where:
 sDF  simplified difficulty function
 D[n]  difficulty estimate after the nth repetition
 Grade  grade scored in the nth repetition
 Rr  expected recall at the time of the nth repetition (based on the Recall[D,S,R] matrix)
Unlike bestfit difficulty, simplified difficulty requires no expensive hillclimbing optimization that may slow down SuperMemo (e.g. when computing learning data for long repetition histories). The accuracy of the simplified difficulty estimation is bound to be less and is currently being estimated.
Startup interval
The first interval Int[1] is determined using the first forgetting curve. Unlike the entries of the Recall[] matrix, the first forgetting curve data is collected using 35 quantiles determined by the interval distribution on a large representative sample collected from an incremental reading collection. Those quantiles span nearly a decade (older SuperMemos collected data for only the first 20 days after memorizing the item). The startup interval (the first optimum interval) is determined by power regression on that forgetting curve data. Power law forgetting is the result of the superposition of exponential forgetting curves of heterogeneous indeterminate difficulty material entering the learning process. The first interval is corrected for the requested forgetting index (rFI) and may be subject to random dispersion that is a standard fixture of SuperMemo algorithms since its inception.
In short, the startup interval is determined by the point in time at which the forgetting curve indicates the recall is likely to fall below the value determined by the requested forgetting index.
Note that the startup interval (with rFI=10%) is not the same as the startup stability. The interval needs to be determined before the first review (i.e. be a resultant of prior repetitions of other items). The startup stability is determined from repetition histories and may differ after each successive repetition. In short, it represents the best fit to data.
Figure: The first forgetting curve (first repetition for items with no lapses). Unlike it was the case in earlier SuperMemos, where all forgetting curves were exponential, the first forgetting curve in SuperMemo 17 is approximated using power regression. This provides for a more accurate mapping due to the heterogeneity of the learning material introduced in the learning process that results in superposition of exponential forgetting with different decay constants. The use of power regression explains why the first interval might be slightly shorter in Algorithm SM17. On a semilog graph, the power regression curve is logarithmic (in yellow), and appearing almost straight. The curve shows that in the presented collection recall drops merely to 58% in four years, which can be explained by a high reuse of the memorized knowledge in real life. In earlier SuperMemos, recall data would only be collected in the span of 20 days, and negatively exponential forgetting curve would make for far lower retrievability predictions. The first optimum interval for the forgetting index of 10% is 3.76 days. The forgetting curve can be described with the formula R=0.987*power(interval,0.07), where 0.987 is the recall on Day 1, while 0.07 is the decay constant. This is case, the formula yields 89.5% recall after 4 days, which is then used as the first rounded optimum interval. Almost 77,000 repetition cases were used to plot the presented graph. Steeper drop in recall will occur in collections with a higher mix of difficult items, in poorly formulated collections, or in new users with lesser mnemonic skills.
Startup stability
Startup stability S[0] is the stability accomplished with the first single review. Unlike stability computed at later repetitions, this stability cannot be derived from the SInc[] matrix. Instead, it is computed in a way similar to the way difficulty is computed. Within a range of possible values, S[0] is used to compute memory stabilities after successive repetitions. The startup stability S[0] is taken as the value that produces the least predicted grade deviation from grades in repetition history.
For histories with perfect recall, infinite stabilities minimize the grade deviation. This is why startup stability needs to be limited to maximum allowed startup stability (S0Max). For all repetition histories in a large dataset, we have plotted the impact of S0Max on the overall grade deviation. It seems that for most users, the deviation does not improve beyond S0Max of 3 months. There are always items that will never fail (e.g. due to a real life repetition), but very few memorized items can survive more than 3 months without the first review.
Reducing S0Max further (e.g. to 1 month), results in SInc[] matrix compensation, which tends to yield similar stabilities after a couple of repetitions. However, lower S0Max results in a higher grade deviation.
Postlapse stability
Postlapse stability (PLS) is the stability resulting from a review with a failing grade. Unlike stability computed after successful repetitions, this stability cannot be derived from the SInc[] matrix.
In the ideal case, for simple memories, forgetting results in a reset of stability back to nearzero. In theory, only difficult items made of composite memories may show a substantial decrease in the costs of relearning, however, even that does not show in data (perhaps due to imperfect difficulty estimations or due to SuperMemo's natural tendency to help users eliminate hardtolearn material in the learning process).
It has been shown long ago that the length of the first postlapse optimum interval is best correlated with the number of memory lapses afforded an item. Even then, postlapse interval usually oscillates in the 14 days range for forgetting index of 10%, and the correlation is not very useful in adding to the efficiency of learning in optimizing the length of that interval. Some competitive spaced repetition software, as well as SuperMemo in its first years, experimented with relearning hypotheses based on ancient wisdom of learning psychology, e.g. by halving intervals after a memory lapse. Current data shows clearly that this is a harmful and timewasting approach.
In addition to memory lapses, there is also a strong correlation between the length of the postlapse interval and retrievability (R). This should be obvious considering that substantial delay in review will result in "undeserved" lapse on items that might be then quite easy to relearn. Algorithm SM17 takes retrievability dimension into account.
In the light of the above, SuperMemo uses a separate matrix for postlapse stabilities: PLS[] with Lapse and Retrievability dimensions. The first interval after scoring a failing grade is then determined as follows:
Int[1]:=PLS[Lapses,R]where:
 Int[1]  first interval (after a failing grade)
 PLS[]  postlapse interval matrix
 Lapses  total number of memory lapses (failing grades) scored by the item
 R  retrievability at the moment of the lapse
Retrievability
Theory
In theory, retrievability R should be easy to derive from stability and interval:
R[n]:=exp(k*t/S[n1])where:
 R[n]  retrievability at the nth repetition
 k  decay constant
 t  time (interval)
 S[n1]  stability after the (n1)th repetition
That neat theoretical approach is made a bit more complex when we consider that forgetting may not be perfectly exponential if items are difficult or with mixed difficulty. In addition, forgetting curves in SuperMemo can be marred by user strategies in all situations where grades are not an ideal reflection of recall/retrievability.
Three measures of retrievability
Theoretical twocomponent model may be undermined by imperfect difficulty filtering, user strategies, implementation shortcomings (e.g. poor choice of parameters), and imperfect modelling of reality. For that reason SuperMemo uses three different measures of retrievability:
 Theoretical retrievability (R) as derived from the exponential decline of recall over time (as above). This measure is a perfect reflection of passing time
 Recall matrix: measured recall R for different levels of item difficulty, memory stability, and theoretical R (or time). This measure is a perfect reflection of average recall (for imperfectly measured memory status parameters)
 RecallGrade correlation: all users have their own average level of recall corresponding with all five grades used in SuperMemo. This measure is highly unreliable (for example, there were cases in which higher grades have been shown to be associated with lower recall)
Recall matrix: Algorithm SM17 accounts for possible nonexponential decay of memory, as well as for D and S estimate errors, by using the Recall[D,S,R] matrix. Recall matrix takes the same arguments as the SInc[] matrix. Recall[D,S,R] is the average recall for items of difficulty D, at stability level S, and at retrievability R. Recall matrix keeps the forgetting curve record with the time axis expressed as retrievability. The main functions of Recall[] matrix are:
 to measure the actual level of recall for different retrievability levels and
 verify the soundness of the model and its parameters (in the ideal theoretical case Recall should equal retrievability).
The actual recall level statistic is used to amend the theoretical retrievability level derived from stability and time.
Figure: The Recall[] matrix graph shows that the actual recall differs from predicted retrievability. For higher stabilities and difficulties, it is harder to reach the desired recall level.
RecallGrade correlation: For individual repetitions, retrievability can be checked against the average recall level registered for individual grades at repetitions. Success and failure of memory, as defined in the grade scale are also used to measure binary recall (0  failure, 1  success). Algorithm SM17 uses both the recallgrade correlation and binary recall to add to the set of information needed to estimate memory stability. Grades carry more information, while binary recall is a better reflection of forgetting. Those two must be weighed up when looking at gradederived estimate of R.
Retrievability estimate
All the three measures of retrievability can be weighed up to provide for the best estimate of R that is used in further estimates of stability and stability increase. Weights used in the estimate will change with inflow of new information (e.g. repetition cases registered in the Recall matrix). The ultimate goal of weight parameters is to minimize the deviation between the model and the grades scored in learning.
A careful distinction must be made between all measures of recall. Theoretical retrievability (R) concept is essential for indexing optimization matrices as R is the ultimate target when converging onto the theoretical model of memory. At the other extreme, stability estimates are itemspecific and need to take into account past recall statistics and actual grades scored by a particular item in the learning process.
Stability
The interval and the resulting grade can be used to draw conclusions about the stability estimate. If the interval is greater than stability, good grades indicate stability underestimate. Conversely, short intervals and poor grades may indicate an overestimate. This type of reasoning underlay the first attempt to compute the stability increase function in this 2005 article. However, due to being applicable to a subset of repetition scenarios, this information is insufficient to estimate stability for all items and all repetitions. In addition, conclusions drawn in 2005 differ substantially from the new data based on far more accurate recallverified retrievability estimate (as described above).
The second, and more reliable source of information about stability is the value derived from retrievability estimate modified along prior recall measurements. Stability can be estimated from retrievability level and the stability estimate value before the repetition. This is the most reliable estimate.
Last but not least, all repetitions produce a new value of estimated stability. That prerepetition estimate is the third source of information on the actual level of stability.
Those three sources of information are weighed up for reliability (e.g. on the number of cases in the Recall matrix used to modify R, or the number of cases in the SInc[] matrix used to generate prerepetition stability). The actual parameters used in weighing up information have been computed using optimization methods targeted at minimizing predicted grade deviation in larger collections.
Stability increase
Stability increase function is used to compute intervals in Algorithm SM17.
The stability increase function is expressed in Algorithm SM17 using three different methods:
 the theoretical formula, which is a multiplicative compound expression of the impact of all its three D, S, R arguments
 the matrix SInc[] built on the basis of data collected from collection's repetition histories
 the best compromise function (BestSInc) that combines the information from the matrix, with support of neighboring matrix entries where data is missing, and with support of the theoretical function where no data is available
Due to its monotonicity, theoretical formula is used in all hillclimbing procedures which are employed richly in the algorithm. It is also used exclusively in new collections where no learning data is available. The SInc[] matrix is the truest expression of stability changes in user's particular collection. The SInc[] matrix has the greatest research value and is used in all contexts where true properties of memory need to be taken into account. The matrix is the key to AlgSM17 adaptability. Finally, the ultimate compromise BestSInc() function is a statement on what is best known about the memory properties and learning strategies of the user and his or her particular collection at any given moment in time.
The choice between the tree expressions of SInc[], and the choice of compromise parameters in BestSInc() are essential for the ultimate efficiency of the algorithm. Those choices and parameters are still in the process of being optimized.
The SInc[] matrix can be computed using three different methods:
 in case of lack of data, from a theoretical universal formula, for all new users
 incrementally during learning while collecting repetition data and executing Algorithm SM17
 wholesale by recomputing stability increase noted in all past repetitions (esp. for users of earlier versions of SuperMemo)
The recommended approach for users of older SuperMemo is to compute a new SInc[] matrix as the first step and then proceed with incremental changes during a course of standard learning with Algorithm SM17.
Interval
Intervals in Algorithm SM17 are computed using stability increase function SInc() (the best compromise variant is used as described above as BestSInc).
The following formula is used:
Int[n]:=S[n1]*SInc[D[n1],S[n1],R[n]]*ln(1rFI/100)/ln(0.9)where:
 Int[n]  interval after the nth repetition
 S[n]  memory stability after nth repetition
 R[n]  memory retrievability at the time of nth repetition
 SInc[D,S,R]  stability increase for a given level of difficulty (D), stability (S) and retrievability (R)
 rFI  requested forgetting index expressed in percent
This formula is analogous to similar formulas in all versions of SuperMemo beginning with Algorithm SM0. The role of the stability increase SInc[] was played earlier by EFactors (based on item difficulty) and OFactors (derived from forgetting curves).
Data
 item repetition histories (date and grade)
 recall matrix: Recall[D,S,R]
 stability increase matrix: SInc[D,S,R] (i.e. the matrix representation of the stability increase function)
The Algorithm: Outline
The following procedure can be used to determine the status of memory (DSR status) at any moment of time for a given history of repetitions. Once the DSR status is known, we can determine the next interval using criteria of choice, e.g. forgetting index, maximization of stability, longterm workload minimization, etc.
 estimate item difficulty D using the history repetitions for that item
 determine startup stability S0 using the history of repetitions
 for all repetition history records repeat the steps below
 compute theoretical retrievability Rt using current stability estimate Sw and the interval Int
 update Recall[] matrix using D, Sw[n1], Rt with the gradederived recall
 compute recallbased retrievability Rr
 compute gradederived retrievability Rg
 estimated weighted Rw from Rt, Rr, and Rg
 compute Rwbased stability Sr
 compute SIncbased Se (Se=Sw[n1]*SInc[D,Sw[n1],Rw])
 compute interval derived stability Si
 estimate weighted Sw from Sr, Se, and Si
 compute the stability increase SInc on the basis of Sw change
 update Sinc[] matrix using D, Sw, Rw with the new SInc value
 compute new interval using Int:=Sw*SInc[D,Sw,Rw]
 go back computing Rt step
Note that replacing Int:=Int[n1]*SInc[D,Sw,Rw] with Int:=Sw*SInc[D,Sw,Rw] frees the user from ever worrying about manual intervention in the length of intervals (e.g. before an exam, or when encountering knowledge in real life, etc.). This is one of the key advantages of Algorithm SM17 over all prior versions of the algorithm.
Metrics: First year of testing
Figure: RMetric graph demonstrates superiority of Algorithm SM17 over the old Algorithm SM15 for the presented collection used in the testing period of 12 months before the release of SuperMemo 17. It was plotted using 10,699 data points (i.e. repetition cases with data from both algorithms), and smoothed up to show the trends. One spot beneath the line of 0 at the vertical axis (Rmetric<0
) corresponds with a moment in time when the previous version of the algorithm appeared superior (in this case, due to a small sample of repetitions). Some positive and negative trends correspond with changes in the algorithm as data were collected in the new algorithm's testing period. A gradual increase in the metric in the months FebMay 2016, might be a statistical aberration, or it might be the result of new interval values and a bigger Rmetric for intervals departing from the optimum used in earlier SuperMemos. The latter interpretation might suggest that the benefits of Algorithm SM17 can gradually increase over time.
Historical note: earlier releases of the algorithm
Although the presented algorithm may seem complex, you should find it easier to follow its steps once you understand the evolution of individual concepts such as EFactor, matrix of optimum intervals, optimum factors, forgetting curves, midinterval repetition, stability, retrievability, etc.
 1985  Paperandpencil version of SuperMemo is formulated (Algorithm SM0). Repetitions of whole pages of material proceed along a fixed table of intervals. See also: Using SuperMemo without a computer
 1987  First computer implementation makes it possible to divide material into individual items. Items are classified into difficulty categories by means of EFactor. Each difficulty category has its own optimum spacing of repetitions (Algorithm SM2)
 1989  SuperMemo 4 was able to modify the function of optimum intervals depending on the student's performance (Algorithm SM4). This was then the first algorithm in which the function of optimal intervals was adaptable
 1989  SuperMemo 5 replaced the matrix of optimum intervals with the matrix of optimal factors (an optimum factor is the ratio between successive intervals). This approach accelerated the adaptation of the function of optimum intervals (Algorithm SM5)
 1991  SuperMemo 6 derived optimal factors from forgetting curves plotted for each entry of the matrix of optimum factors. This could dramatically speed up the convergence of the function of optimum intervals to its ultimate value (Algorithm SM6). This was then the first adaptable algorithm that would use regression to find the best fit to the actual memory performance data. Unlike SuperMemo 5, which could keep converging and diverging depending on the quality of the learning material and the learning process, SuperMemo 6 would get closer to the student's ultimate memory model with each day of learning
 1995  SuperMemo 8 capitalized on data collected by users of SuperMemo 6 and SuperMemo 7 and added a number of improvements that strengthened the theoretical validity of the function of optimum intervals and made it possible to accelerate its adaptation, esp. in the early stages of learning (Algorithm SM8). New concepts:
 replacing EFactors with absolute difficulty factors: AFactors. Item difficulty was thus defined in terms of actual properties of human memory, and would not depend on the average difficulty of the learning material
 fast approximation of AFactors from the First Grade vs. AFactor correlation graph and Grade vs. Forgetting index graph. This makes it possible to quickly guess item's difficulty before more data is available
 realtime adjustment of the matrix of optimal factors based on the power approximation of the decline of optimum factors
 2002  SuperMemo 2002 introduced the first SuperMemo algorithm that is resistant to interference from delay or advancement of repetitions. This makes it possible to safely delay repetitions (Postpone) or advance repetitions (Review):
 accounting for delayed repetitions by introducing the concept of repetition categories
 accounting for advanced repetitions by introducing OFactor decrements derived from the concept of the spacing effect
 2005  SuperMemo 2004 introduced boundaries on A and B parameters computed from the Grade vs. Forgetting Index data. This plugs up a weakness in the algorithm that showed when importing repetitions from other applications (e.g. open source MemAid). If a large number of easy repetitions occurred at unnaturally long intervals (as after pretraining with another application), the graph might show reversed monotonicity that would temporarily affect the estimation of AFactors (the speed of selfcorrection would be reversely proportional to the amount of flawed data). When boundaries are imposed, selfcorrection is instant, and the accuracy of AFactor estimation increases with each repetition executed in SuperMemo
 2011  with Algorithm SM15, SuperMemo 15 eliminated two weaknesses of Algorithm SM11 that would show up in heavily overloaded collections with very large item delays:
 UFactors now allow of correctly interpreting repetition delays of up to 15 years (previously 60 days only)
 forgetting curves are now corrected for repetition delays beyond the maximum registered UFactor value (preventing failed grades in delayed repetitions decreasing the estimates of the optimum interval for standardlyscheduled items in the same category)
 2015  Algorithm SM17 is a successor to all prior versions of the algorithm in future SuperMemos. It has passed all important benchmarks, and was set to be part of SuperMemo 17 in 2016 (as well as other SuperMemos later on)