Only what the research actually supports.

The forgetting curve and the memory model (FSRS-4.5)

Every item carries a stability S (the number of days until the probability of recall falls below 90%) and a difficulty D. The probability of recall falls along a power-law forgetting curve:

R(t) = (1 + 1981 · tS)^−0.5

FSRS is an open model fitted to hundreds of millions of real reviews, closer to the data of modern learners than Ebbinghaus’s classical curve. S grows with every successful review, and it grows most when the recall was only just successful.

Retrieval practice

Recalling something strengthens memory in a way that rereading does not: one of the more robust findings in cognitive psychology. Ebbio is built so that you always bring the answer to mind before rating yourself with ○ △ ×.

The spacing effect

The next review is scheduled for the moment recall falls to the target retention (90% by default, adjustable in three steps or in 1% increments, and settable per collection or per item). After each success the interval widens exponentially: one day, four days, two weeks, and on.

Desirable difficulties

The harder the successful recall, the larger the gain. FSRS’s update equations put the maximum efficiency at the difficulty where you only just manage it. Items you missed reappear at the end of the session, so the day always ends on a successful recall.

Workload smoothing

When reviews pile onto one day, Ebbio does not simply postpone them. It redistributes within about 15% of each item’s interval, up to 10 days, and never moves fragile memories with intervals under three days. It moves the most stable items first and chooses the destination by a utilisation figure that accounts for different limits on different weekdays.

Goal-date scheduling

Set a date you need to be ready for and Ebbio works backward along the forgetting curve to find the last review that satisfies the target retention (95% by default). It avoids both the fragility of cramming and the blindness of ordinary spaced study to a deadline.

Sources

The Ebbinghaus forgetting curve

Hermann Ebbinghaus, Über das Gedächtnis: Untersuchungen zur experimentellen Psychologie (1885). The origin of the study of forgetting, and the origin of Ebbio’s name.

FSRS (Free Spaced Repetition Scheduler)

The open spaced-repetition algorithm published by the open-spaced-repetition project (github.com/open-spaced-repetition), with parameters optimised against hundreds of millions of real review logs. Ebbio implements the published algorithm and equations independently and does not include the project’s own code.

Retrieval practice

Roediger & Karpicke, “Test-Enhanced Learning: Taking Memory Tests Improves Long-Term Retention” (Psychological Science, 2006).

The spacing effect

Cepeda et al., “Distributed Practice in Verbal Recall Tasks: A Review and Quantitative Synthesis” (Psychological Bulletin, 2006).

Desirable difficulties

The concept proposed by Robert A. Bjork: the greater the effort of a successful retrieval, the stronger the memory. Built into the FSRS update equations.