Ease Factor

Definition:

Ease Factor (also called the E-Factor or interval modifier) is a numerical multiplier assigned to each flashcard in the SM-2 spaced repetition algorithm. It determines how aggressively a card’s review interval grows after each successful recall. A higher Ease Factor means longer gaps between reviews; a lower one means more frequent repetition.

In-Depth Explanation

The Ease Factor controls how aggressively a card’s interval grows with each successful review. It is the central per-card parameter in the SM-2 algorithm — each card accumulates (or loses) Ease Factor independently based on learner ratings. Cards that are frequently answered with Hard or Again become low-Ease cards reviewed very often; cards consistently rated Easy develop high Ease and long intervals.

How Ease Factor Works

In SM-2, the next interval for a card is calculated as:

> New Interval = Current Interval × Ease Factor

For example, if a card has an interval of 10 days and an Ease Factor of 2.5, the next review is scheduled in 25 days. If the Ease Factor drops to 1.8, the next interval would be only 18 days.

Default and Range

• Starting value: 2.5 (Anki default)
• Minimum value: 1.3 (Anki enforces this floor to prevent cards from being reviewed constantly)
• No hard maximum (though values above 3.0 are uncommon in practice)

Each time a card is rated:

• Again (0): Ease Factor decreases by 0.20
• Hard (1): Ease Factor decreases by 0.15
• Good (2): Ease Factor unchanged
• Easy (3): Ease Factor increases by 0.15

The “Ease Hell” Problem

A well-known issue with SM-2/Anki is what learners call Ease Hell: repeatedly pressing “Hard” on a difficult card drives its Ease Factor down to the minimum (1.3). Once there, intervals grow barely at all — the card gets reviewed again and again with little spacing, defeating the purpose of SRS.

For example, a card stuck at Ease Factor 1.3 with a current interval of 30 days would schedule its next review at only 39 days (30 × 1.3), instead of 75 days at the default 2.5. Over months, such cards pile up and dominate review sessions.

Common fixes:

• Use the FSRS algorithm, which replaces Ease Factor with a more dynamic stability/difficulty model.
• Use the ResetEZ or No Penalties, No Rewards Anki add-ons to prevent Ease Factor from changing.
• Regularly use the Ease Factor Reset to return cards to 2.5.
• Press “Good” instead of “Hard” — many Anki veterans recommend this to avoid Ease Hell entirely.

Ease Factor vs. FSRS Difficulty

FSRS (Free Spaced Repetition Scheduler) does away with Ease Factor as a direct multiplier. Instead, it models each card’s:

• Difficulty (D): A 1–10 scale representing how intrinsically hard the card is to remember.
• Stability (S): How many days until the card has a 90% chance of being recalled.

FSRS’s difficulty score affects scheduling in a more nuanced way than SM-2’s Ease Factor, and it can be recalibrated from the full review history rather than being altered by individual button presses. This makes it more resistant to Ease Hell-type degradation.

Practical Application

• If using SM-2: Press “Good” for most correct cards. Reserve “Easy” for cards you find genuinely trivial. Avoid “Hard” unless absolutely necessary to prevent Ease Hell drift.
• If using FSRS: Ease Factor is replaced — focus on accurate Again/Good ratings rather than gaming the modifier.
• Monitor your average Ease Factor in Anki Stats. If the average drops below 2.0, your deck may need curation — cards might be too difficult or poorly written.

History

The Ease Factor was introduced by Piotr Wozniak as a core component of the SM-2 algorithm (1987), the spaced repetition formula used in SuperMemo and subsequently adopted by Anki. SM-2 was designed to estimate the optimal review interval for each memorized item individually, using the Ease Factor as the per-item difficulty multiplier. Damien Elmes incorporated SM-2 into Anki (2006) with the default Ease Factor set to 2.5, a value derived from Wozniak’s original calibration data. The Ease Factor became widely discussed in the SRS community as the algorithm’s most user-visible tuning mechanism. By the 2020s, significant evidence had accumulated that SM-2’s Ease Factor mechanism had systematic problems (particularly “Ease Hell”) that newer algorithms like FSRS addressed by replacing the static multiplier with dynamically learned stability and retrievability parameters.

Common Misconceptions

“A higher Ease Factor always means better learning.” An artificially high Ease Factor (inflated by pressing “Easy” on cards you actually only barely know) produces longer intervals than retention data supports, leading to more forgetting. A genuinely high Ease Factor on a well-known card is appropriate; a high Ease Factor maintained by gaming the ratings is counterproductive.

“Ease Factor can be manually reset to improve a damaged deck.” While Anki allows Ease Factor to be inspected and adjusted via add-ons, simply resetting Ease Factor to 2.5 does not fix underlying issues — if cards are genuinely hard, the Ease Factor will drift down again quickly. The solution to an entrenched low-Ease deck involves either improving card design, reducing deck size, or switching to FSRS.

Criticisms

The SM-2 Ease Factor mechanism has been criticized extensively in SRS research and the Anki community. The “Ease Hell” problem — where cards become trapped with low Ease Factors after several Hard or Again ratings, leading to excessive review frequency that persists even after mastery is recovered — is a structural flaw in the SM-2 design. The fixed per-rating increment/decrement values for Ease Factor changes do not adapt to individual learner characteristics. Research by Jarrett Ye and others demonstrated that FSRS (Free Spaced Repetition Scheduler), which replaces Ease Factor with stability and difficulty parameters learned from actual review patterns, produces better retention outcomes with fewer reviews.

Social Media Sentiment

Ease Factor and “Ease Hell” are actively discussed in the Anki community on Reddit (r/Anki, r/LearnJapanese, r/medicalschool). The community broadly understands that pressing “Easy” too often inflates Ease Factors while pressing “Hard” creates Ease Hell — and the advice to “press Good for most cards” is a community standard. The transition to FSRS (now the default algorithm in Anki 23.10+) has reduced the relevance of Ease Factor for new users, but discussions comparing SM-2 and FSRS mechanics regularly reference Ease Factor as the SM-2 parameter FSRS was designed to replace.

Last updated: 2026-04

Related Terms

• SM-2 Algorithm — the scheduling system where Ease Factor originates
• FSRS — the modern replacement that models memory more accurately
• Retention Rate — the outcome metric that Ease Factor indirectly controls
• Spaced Repetition System — the broader practice

Research

Wozniak, P. A. (1990). SuperMemo 2: Implementation of a method for learning foreign vocabulary. SuperMemo World.

The original documentation of the SM-2 algorithm by its creator, defining the Ease Factor mechanism and the interval multiplication formula — the primary source for understanding what Ease Factor is, where it came from, and why the SM-2 design choices were made.

Settles, B., & Meeder, B. (2016). A trainable spaced repetition model for language learning. Proceedings of the 54th Annual Meeting of the Association for Computational Linguistics, 1848–1858.

An empirical study of spaced repetition modeling on Duolingo learner data, proposing adaptive alternatives to the fixed SM-2 Ease Factor mechanism — one of the data-backed arguments for moving beyond the hard-coded Ease Factor approach toward learner-specific interval optimization.

Ye, J. (2022). FSRS4Anki: A new algorithm for Anki. GitHub.

The open-source repository documenting the FSRS algorithm that replaces SM-2’s Ease Factor with a machine-learning-based difficulty and stability model — the definitive technical reference for understanding what modern SRS research proposes as an alternative to the Ease Factor mechanism.

See Also

• Sakubo