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 SM-2 Algorithm

The scheduling system is based on the SM-2 algorithm, originally developed for the SuperMemo project. SM-2 is one of the most widely studied and validated spaced repetition algorithms. It has been in use since the late 1980s and remains effective for long-term memorization tasks.

Species uses a variation of the SM-2 algorithm because it is open, well-documented, and proven across decades of real-world use. More recent proprietary algorithms exist, but their inner workings are not publicly available for verification.

How Intervals Are Calculated

Each review card has an interval — the number of days until its next review. After each review, the new interval is calculated based on your rating:

• Good — The current interval is multiplied by the card's ease factor. For example, a card with a 10-day interval and an ease factor of 2.5 would be scheduled for review in 25 days.
• Easy — The interval is multiplied by the ease factor with an additional bonus, producing an even longer interval.
• Hard — The interval is multiplied by a reduced factor (1.2), resulting in a shorter interval than "Good."
• Again — The card enters relearning and its interval is significantly reduced.

A global interval modifier is applied to all graduated intervals to fine-tune the overall pace of the schedule.

Ease Factor

Every card has an ease factor that represents how difficult you find that specific card. The default ease factor is 2.5. It adjusts based on your ratings:

The ease factor has a minimum value of 1.3 to prevent intervals from becoming impractically short. Over time, the ease factor stabilizes for each card based on your consistent performance with it.

Cards that you find genuinely difficult will accumulate a lower ease factor, causing them to appear more frequently. Cards that you consistently recall with ease will develop a higher ease factor and appear less often.

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 you retained the material. The next interval will be calculated based on the actual elapsed time, not the originally scheduled interval.
• If you fail to recall an overdue card, it enters relearning with a reduced interval, 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 on Review Cards