Forgetting Curve

The forgetting curve is one of the foundational discoveries in the science of memory. In 1885, German psychologist Hermann Ebbinghaus conducted rigorous self-experiments memorising nonsense syllables and tracking how quickly he forgot them. He found that memory decay follows a predictable exponential pattern: without review, roughly 50% of new information is lost within an hour, 70% within a day, and 90% within a week. The exact rates vary by individual and material, but the exponential shape is remarkably consistent.

The critical insight from the forgetting curve is that each successful review resets and flattens the curve. After the first review, the decay is slower; after the second, slower still. This is the entire scientific basis for spaced repetition — by timing reviews at the point where memory is about to drop below a threshold, you can maintain high retention with progressively less effort. Each review pushes the next optimal review point further into the future.

Revu's FSRS algorithm models the forgetting curve for every individual card using a stability parameter that increases with successful reviews and a difficulty parameter that captures how inherently hard each item is for you. Rather than using fixed intervals, FSRS calculates precisely when your recall probability will drop to the target threshold (typically 90%) and schedules the review for that moment. This approach fights the forgetting curve with mathematical precision, ensuring you review at the optimal time — not too early (wasting time) and not too late (losing the memory).