Difficulty
Difficulty refers to how challenging it is to learn or remember a particular piece of information or item. It reflects the complexity or intrinsic hardness of the memory. Items considered difficult require more effort or repetitions to memorize and retain. Difficulty can be influenced by how complex or unfamiliar the information is.
Unlike the other two, it is often estimated through heuristics or the learner’s self-evaluation rather than a strict mathematical definition.
Stability
Stability measures the durability of a memory over time. It defines how long a memory will last if it is not retrieved or rehearsed. High stability means the memory decays slowly and persists for a long duration. Stability determines the spacing intervals between reviews in spaced repetition systems—more stable memories can be reviewed less often.
Mathematically, stability represents the timespan over which the probability of recall remains above a threshold (e.g., 90%).
Retrievability
Retrievability is the current probability or likelihood of successfully recalling a memory at a given point in time. It changes dynamically depending on how much time has passed since the last review or retrieval. Retrievability typically declines exponentially after learning or review and is what learners try to boost by repeating or recalling the item.
High retrievability means easy and confident recall; low retrievability means the memory is weak or hard to access.
In summary
| Component | Description | Role in Memory Process |
|---|---|---|
| Difficulty (D) | How hard it is to learn or remember an item | Initial learning challenge and complexity |
| Stability (S) | How durable a memory is over time | How long memory lasts without review |
| Retrievability (R) | Likelihood to recall the memory now | Current recall strength, changes with time |
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