What is FSRS and Why Does It Replace SM-2?
For over 15 years, Anki ran on SM-2, an algorithm developed by Piotr Wozniak in 1987. While revolutionary in its time, SM-2 suffers from severe mathematical limitations when applied to massive 30,000-card medical school decks like AnKing:
- Ease Hell: In SM-2, pressing "Hard" permanently reduces a card's Ease factor by 15%. Over months of studying, hundreds of cards get permanently stuck in short intervals, creating an unmanageable review treadmill.
- Rigid Interval Multipliers: SM-2 assumes all humans forget at identical linear rates, regardless of card complexity or individual memory stability.
FSRS (Free Spaced Repetition Scheduler) replaces heuristics with a modern, two-component model of memory (Stability and Retrievability) backed by machine learning. It models how memory decays over time and dynamically calculates the exact interval needed to achieve your chosen retention rate.
The Single Most Important Setting: Desired Retention
In SM-2, you had no direct control over how well you retained cards; you simply adjusted "interval modifiers" blindly. Under FSRS, you specify your exact target retention rate via Desired Retention (DR):
- 0.90 (90% Retention): The consensus recommendation across r/medicalschoolanki for dedicated USMLE study. You retain 9 out of 10 cards on review.
- 0.85 (85% Retention): The optimal efficiency setting for clinical rotations and shelf exams. Dropping target retention from 90% to 85% reduces total daily review count by approximately 30% to 35%, giving you critical hours back for UWorld questions and clinical duties.
- 0.95+ (95% Retention): The "Diminishing Returns Trap." Pushing retention from 90% to 95% nearly doubles your daily review workload for a marginal 5% gain in recall. Avoid this setting.
Recommended FSRS Settings for Medical School
| Parameter | Recommended Value | Why |
|---|---|---|
| Desired Retention | 0.88 - 0.90 | Sweet spot between memory retention and daily review volume. |
| Learning Steps | 10m or 15m | Single intra-day step. FSRS handles Day 1+ scheduling automatically. |
| Relearning Steps | 10m | Quick reset without artificially restarting multi-day lapses. |
| Maximum Interval | 180 - 365 days | Caps intervals so high-yield cards are seen before your board exam. |
The Golden Rule of Anki Buttons Under FSRS
Under FSRS, the 4 rating buttons have distinct mathematical meanings. Misusing them corrupts your personalized weights:
- Again (1): Complete lapse. You could not recall the answer.
- Hard (2): Successful recall, but required noticeable effort or took longer than expected. Crucial: In FSRS, "Hard" is a passing grade, not a failing grade!
- Good (3): Normal recall with standard effort. This should be your default button for 75%+ of cards.
- Easy (4): Instantaneous recall. Reserves the card for a longer interval leap.
Verify Your Deck with Penziv
Unsure whether your current retention matches your FSRS target? Use the Anki FSRS Target Retention Calculator to simulate time savings, or upload your collection directly to the Penziv Deck Analyzer to verify your True Retention across every clinical subject.