# Glossary:Recall matrix

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- Recall matrix (Recall[])
- the matrix that shows how recall depends on retrievability (R). In an ideal case, recall and retrievability should be the same, however, there are factors that make it hard to achieve. For example, difficult items produce non-exponential forgetting curves, SuperMemo finds it hard to sort items by difficulty, difficulty is fluid, etc. The
**Recall matrix**is then used to predict recall on the basis of retrievability for items with varying difficulty and stability. The matrix can be used to produce the approximation of the recall function which helps estimating recall even if no recall data is available. The**Recall matrix**can be compared to the forgetting curve matrix. In older algorithms, forgetting curve matrix was used with time measured in U-Factor unit. The**Recall matrix**measures time in retrievability units. You can inspect the**Recall matrix**using:**Toolkit : Memory : 4D Graphs : Recall**