Scheduling System

Species uses a spaced repetition algorithm to determine when each card should be reviewed. The algorithm adapts to your performance on every card, scheduling frequent reviews for difficult material and infrequent reviews for material you know well.

The FSRS Algorithm

The scheduling system is powered by FSRS (Free Spaced Repetition Scheduler), a modern spaced repetition algorithm based on a mathematical model of human memory. Unlike older systems that rely on fixed multipliers, FSRS uses three core variables — Difficulty, Stability, and Retrievability — to predict when you are most likely to forget each card, and schedules reviews accordingly.

FSRS was developed through large-scale analysis of real review data and has been shown to produce more accurate scheduling than earlier algorithms. Its parameters are open and well-documented.

How Intervals Are Calculated

Each card has a stability value — the estimated number of days you can go before your recall probability drops below the target threshold (90%). After each review, FSRS recalculates stability based on your rating:

• Good — Stability increases substantially. The card is scheduled further into the future.
• Easy — Stability increases by the largest amount, producing the longest interval.
• Hard — Stability increases by a smaller amount, resulting in a shorter interval than "Good."
• Again — Stability is reset to a low value. The card will be reviewed again soon.

The resulting review interval is derived directly from the updated stability, ensuring that each card is shown at the point where reviewing it has the greatest benefit for long-term retention.

Difficulty

Every card has a difficulty rating (on a scale of 1 to 10) that represents how challenging you find that specific card. New cards start at a moderate difficulty. It adjusts based on your ratings:

Cards that you find genuinely difficult will accumulate a higher difficulty, causing FSRS to assign shorter intervals — you will see them more often. Cards that you consistently recall with ease will develop a lower difficulty and appear less frequently.

Stability and Retrievability

Stability is the central scheduling metric. It represents the number of days after which your probability of recalling the card drops to 90%. A stability of 30 means the algorithm estimates you have a 90% chance of remembering the card 30 days after your last review.

Retrievability is your estimated recall probability at any given moment. It decreases over time since your last review, following a predictable forgetting curve. When retrievability drops near the 90% threshold, the card becomes due for review.

This probability-based approach means that scheduling decisions are grounded in a model of how memory actually works, rather than relying on fixed rules.

What Happens with Overdue Cards

If you miss a scheduled review and a card becomes overdue, the algorithm accounts for the elapsed time when you eventually review it:

• If you correctly recall an overdue card, the algorithm considers the longer-than-planned interval as evidence that your memory is stronger than expected. Stability increases accordingly, and the next interval will reflect this.
• If you fail to recall an overdue card, stability is reset and the card re-enters short intervals, the same as any other lapse.

This means that returning after a break does not permanently damage your progress. Cards you still remember will be rescheduled appropriately, and cards you have forgotten will be relearned.

Summary of Rating Effects