A-Factor

What is the A-Factor?

The A-Factor (short for Absolute Difficulty Factor) is a number assigned to each element in SuperMemo. It plays a key role in determining how fast intervals between repetitions grow as you learn.

The higher the A-Factor, the faster the interval growth

Do Beginners Need to Understand A-Factors?

If you're using the latest version of SuperMemo, you don’t need to worry about A-Factors. The algorithm handles them automatically.

A-Factors determine how quickly your review intervals increase.

A-Factor: Items vs. Topics

• For items (e.g. flashcards):
  • A-Factor reflects how easy the item is
  • Easier items → higher A-Factor → longer intervals

• For topics, tasks and concepts (e.g. articles):
  • A-Factor acts as a multiplier for interval growth, determining how often a topic is reviewed
  • Important topics → lower A-Factor → shorter intervals

How Is the A-Factor Calculated?

Items

The A-Factor is defined as:

 A-Factor = Second optimal interval / First optimal interval

A-factors range from 1.2 (for difficult items) to 6.9 (for easy items)

In Algorithm SM-15: The initial value of A-Factor is derived from the first grade obtained by the item, and the correlation graph of the first grade and A-Factor (G-AF graph). This graph is updated after each repetition in which a new A-Factor value is estimated and correlated with the item's first grade. Subsequent approximations of the real A-Factor value are done after each repetition by using grades, OF matrix, and a correlation graph that shows the correspondence of the grade with the expected forgetting index (FI-G graph). The grade used to compute the initial A-Factor is normalized, i.e. adjusted for the difference between the actually used interval and the optimum interval for the forgetting index equal 10%

Topics, tasks and concepts

The A-Factor is used to compute the next interval:

 Next interval = Current interval * A-Factor

Historical Context and Evolution

A-Factors evolved from E-factors in SuperMemo 1.0.

• in Algorithm SM-2, E-factors were modified slightly in each repetition depending on the grade. Bad grades resulted in reducing the E-Factor and increasing the number of repetitions
• in Algorithms SM-4, SM-5 and SM-6, E-factors were used to index optimization matrices, and depended on the difficulty distribution in the collection. This means, E-factors were relative to the difficulty of the collection
• Algorithm SM-8 replaced E-factors with absolute difficulty factors: A-Factors. All collections had the same definition of difficulty that was dependent only on the difficulty of the item for the user with his prior knowledge at any given moment of time. A popular Anki myth speaks of general item difficulty that meaningfully reflects difficulty of an item for many users. A-Factors were used in the same way by Algorithm SM-8, Algorithm SM-11 and Algorithm SM-15
• newer SuperMemos use their own definition of difficulty, however, they still use A-factors in the Algorithm SM-15 that runs in the background (e.g. to measure the efficiency of the new algorithm)

Anki users may think of A-Factor as a cousin of Ease derived from E-factors in Algorithm SM-2.

Using A-Factors

• To see or change an element's A-Factor, use either the Element Priority or Element Parameters window in SuperMemo. Just remember: only topics, concepts, and tasks allow you to change the A-Factor.
• You can view an element's A-Factor in the Element Data window.

See also

• Algorithm SM-2 - introduced E-Factors (in SuperMemo 1.0)
• Algorithm SM-8 - introduced A-Factors
• Algorithm SM-15 - still uses A-Factors, and is still used in new SuperMemos
• Algorithm SM-17 - A-factors for items are still used in the background in Algorithm SM-15
• Algorithm SM-18 - A-factors for items are still used in the background in Algorithm SM-15
• Element Parameters dialog - displays the A-Factor and allows of its change
• Element data window - displays the A-factor of the displayed element
• A-Factor_distributions_may_differ_wildly
• A-Factor: texts refactoring (video)

This glossary entry is used to explain SuperMemo, a pioneer of spaced repetition software since 1987