# Retrievability

**Retrievability of memory** is a variable of long-term memory that corresponds with the *memory clock*. It expresses the probability of recall after a given interval in time. Internally, retrievability may determine the increase in memory stability via re-activation (see: spacing effect). Outwardly, *memory clock* will show as a decline in the probability of recall. The name comes from the fact that the longer the time since the last review, the less the chance of retrieving the memory. We can map retrievability with the help of the forgetting curve.

**Stability (S)** and **Retrievability (R)** are part of the Two component model of long-term memory.

This glossary entry is used to explain SuperMemo, a pioneer of spaced repetition software since 1987

Figure:The first forgetting curve for newly learned knowledge collected with SuperMemo. Power approximation is used in this case due to the heterogeneity of the learning material freshly introduced in the learning process. Lack of separation by memory complexity results in superposition of exponential forgetting with different decay constants. On a semi-log graph, the power regression curve is logarithmic (in yellow), and appearing almost straight. The curve shows that in the presented case recall drops merely to 58% in four years, which can be explained by a high reuse of memorized knowledge in real life. The first optimum interval for review at retrievability of 90% is 3.96 days. The forgetting curve can be described with the formula R=0.9906*power(interval,-0.07), where 0.9906 is the recall after one day, while -0.07 is the decay constant. In this is case, the formula yields 90% recall after 4 days. 80,399 repetition cases were used to plot the presented graph. Steeper drop in recall will occur if the material contains a higher proportion of difficult knowledge (esp. poorly formulated knowledge), or in new students with lesser mnemonic skills. Curve irregularity at intervals 15-20 comes from a smaller sample of repetitions (later interval categories on a log scale encompass a wider range of intervals)