The Mechanics of Ease Hell in SM-2
Under Anki's default legacy SM-2 algorithm, every card starts with an Ease Factor (EF) of 250% (2.50). When you review a mature card and press "Good", Anki calculates its next interval by multiplying the previous interval by that Ease Factor:
Next Interval = Current Interval × Ease Factor
For example, a card with a 10-day interval and 250% Ease graduates to 25 days on "Good".
The trap lies in the "Hard" button. In SM-2, pressing "Hard" multiplies the interval by a modest 1.2, but it applies an irreversible structural penalty: it subtracts 0.15 EF units (15 percentage points) from the card's Ease Factor. Pressing "Again" penalizes the card by 0.20 EF units (20 percentage points).
If you press "Hard" or "Again" a few times during challenging modules, the card's Ease drops to the algorithm's hard floor of 130%. At 130%, a 10-day interval only expands to 13 days on "Good", 17 days on the next review, and 22 days after that. The card enters a state of near-daily repetition regardless of how many times you pass it. You can review the underlying mathematics in the official Anki Manual on Deck Options.
The Real-World Symptoms
- Review Queue Bloat: Your daily review count climbs steadily even though you are not adding new cards.
- Young Card Plateau: Cards cycle perpetually between 1 and 14 days, never maturing into true long-term stability.
- Cognitive Fatigue: You spend 60% of your daily study time answering cards you already know, but that the algorithm refuses to space forward.
The Permanent Cure: Migrate to FSRS
Legacy add-ons treated the symptoms of Ease Hell; FSRS (Free Spaced Repetition Scheduler) eradicates the root cause. As documented in the Open Spaced Repetition repository, FSRS replaces arbitrary multipliers with empirical memory modeling.
FSRS models human memory using three independent variables: Difficulty (D), Stability (S), and Retrievability (R):
- Difficulty is Decoupled from Interval Growth: In FSRS, card difficulty is tracked as an inherent property between 1 and 10. It does not compound downward in a vicious cycle.
- Intervals Derive from Memory Retrievability: Instead of multiplying an arbitrary Ease Factor, FSRS uses a power-law forgetting curve to calculate the exact calendar day your memory retrievability will drop to your target retention (e.g., 90%).
When you enable FSRS in Anki (Tools → Deck Options → Advanced → Enable FSRS), the Ease Factor is completely bypassed during scheduling. Legacy add-ons like Straight Reward and Reset Ease should be uninstalled, as they interfere with FSRS's machine-learned weights.
The Two-Button Paradigm: Fix Your Rating Habits
Medical students frequently cause their own review debt by treating "Hard" as a polite way to give themselves partial credit on cards they actually failed. In spaced repetition, memory is binary: you either recalled the clinical concept or you did not.
- Press Again (1): If you could not recall the mechanism, took more than 10 seconds of hesitation, or recognized only part of the card.
- Press Good (3): If you successfully recalled the diagnostic fact within normal effort. Good should account for the vast majority of your passes.
- Reserve Hard (2) and Easy (4): Hard is a successful pass that required severe strain; Easy is immediate instant recall. Overusing Hard degrades interval spacing.
Remediation Options if Staying on SM-2
If you cannot migrate to Anki 23.10+ and must remain on SM-2, follow this recovery protocol:
1. Selective Ease Resetting (Not Schedule Resetting)
Do not select "Set Due Date" or "Forget" on your cards. A full schedule reset nukes months of memory stability history, forcing you to relearn thousands of mature cards from Day 1.
Instead, open the Anki Browser, filter for cards with low ease (e.g., prop:ease<1.8), and use the Reset Ease feature (or an ease-reset add-on) to restore their starting Ease Factor to 250% while preserving their current intervals.
2. The Straight Reward Add-on Mechanism
The Straight Reward add-on modifies SM-2 by rewarding consecutive successful reviews. When you hit a streak of consecutive "Good" or "Easy" answers (typically 3 to 4 in a row), the add-on adds a small bonus (e.g., +0.05 EF units) back to the card's Ease Factor. This provides a gradual rehabilitation path for cards stuck at 130% Ease without manual intervention.