Memory Science: Forgetting Curves & Spacing Intervals

$$R(t)=e^{-\frac{t}{S}}$$
Where $R(t)$ is retention probability at elapsed time $t$, and $S$ is memory stability (half-life of the memory trace).

The empirical finding that learning is far more durable when study sessions are spaced out over time rather than crammed into a single massed session of equal duration.

Memory stability $S$ increases, meaning the retention curve flattens. Forgetting decelerates dramatically, requiring progressively longer intervals before the next review.

Actively retrieving knowledge from memory produces substantially higher long-term retention than passive re-reading or re-exposure to the same material for equal time.

Conditions that make initial learning feel slower and more difficult (such as spacing, testing, and interleaving) paradoxically trigger deeper cognitive processing that maximizes long-term retention and transfer.

When a memory is difficult to retrieve (just before forgetting), the effort required to reconstruct it triggers a much greater increase in subsequent storage strength than retrieving an easily accessible item.

A multiplier assigned to each card reflecting its difficulty. When a card is rated easy, the factor increases (stretching future intervals); when rated difficult, the factor decreases (tightening intervals).