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Strength was a measure Duolingo used to indicate which learned words and skills a learner currently most needs to practice—the lower an item's strength, the more it is in need of practice[1]. Currently strength is not shown on Duolingo, but it can be seen in Duome / Individual progress page.

An item's strength decays over time when it is not practiced. The rate of this decay is calculated based on factors such as how much one has already practiced the item and how well one knows the word[1].

Strength and decay rate together constitute the two main components of Duolingo's spaced repetition model.[2][3]

Word strength[]

The Words section showing the strengths of individual words (the blue button doesn't exist anymore)

Word strength is an indication of how ready a word is to be practiced again based on how long ago it was last practiced and its decay rate.[3] Practice sessions used to choose some of the weakest words for practice.[3] This is probably discontinued because the button "Practice weakest words" is currently not available.

The strengths of the learned words can be viewed in the Words section (in courses where it is available).

Skill strength[]

A skill with low strength (discontinued feature)

Skill strength is a function of the strengths of the words taught within the skill.[4] It was indicated on a semicircular meter on the left side of a completed skill.

Course strength[]

The Strengthen skills button, with the course strength meter on the left edge.

Course strength is a function of the strengths of all your learned words and skills in the course. Again, it is possible for a course to be at "full strength" even if not all words or skills are.


A skill with a comically fast decay rate (illustration purposes only).

Decay is a discontinued feature that refers to the weakening of an item's strength over time when the user doesn't practice it. The rate at which an item's strength decays depends on how well and often the user has practiced it.[5][6]

Word decay[]

The decay rate of a word depended on several factors:[3]

  • Accuracy - the correctness of the user's responses during lessons and practices;[3]
  • Consistency - how consistently the user gets the word right;[1]
  • Peeking - the number of times the user "peeks" at the meaning of a word during lessons and practices;[3]
  • Practice frequency - the number of times the user practices the word;
  • Time - the time since the user last practiced the word;
  • Type of mistake - mistakes made are rated differently (e.g. noun gender vs spelling mistake)[1];
  • Word difficulty - the difficulty of each word[1]; and
  • Other metrics - metrics known only to Duolingo.

The better the learner seems to know a word based on the above criteria, the more slowly that word's strength will decay, and therefore the longer it will be before the learner is prompted to practice it again. Conversely, the worse the learner seems to know a word based on those criteria, the faster its strength will decay and the sooner they will be prompted to practice it again.

Skill decay[]

As skill strength is a function of the strengths of the skill's individual words,[4] skill decay is in turn a function of the decay rates of the skill's individual words.

Strength versus decay rate[]

Strength and decay rate serve two different purposes in Duolingo's spaced repetition model. Strength is used to indicate which words most need practicing right now,[6] while decay rate is used to affect how strengths change over time in order to take advantage of the spacing effect.[5] According to Duolingo staff member Burr Settles:[7]

Words['s strengths] are bumped to 4 bars whenever you learn or practice them, but the decay rate depends on how often you get them right or wrong (so new words decay in a day or two, words you've gotten right 10+ times might take months).

See also[]

External links[]


  1. 1.0 1.1 1.2 1.3 1.4 Quora, How does Duolingo determine the "strength" of skills?, Brendan Meeder, February 22, 2014.
  2. Official Duolingo Blog, "A New Duolingo for iOS 7", October 3, 2013. Last retrieved 2019-05-21.
  3. 3.0 3.1 3.2 3.3 3.4 3.5 Duolingo Forum, "Notice some changes in word strength?", Burr Settles (tatou), June 12, 2013.
  4. 4.0 4.1 Comment by Burr Settles (tatou), February 12, 2014 in forum post "German Tree Bugged".
  5. 5.0 5.1 Comment by Burr Settles (tatou), July 10, 2013, in forum post "What Language Acquisition Theory(s) Is Used in Duolingo?".
  6. 6.0 6.1 Comment by Burr Settles (tatou), November 4, 2013, in forum post 'Does "lesson practice" function has to be close to useless?'.
  7. Comment by Burr Settles (tatou), September 28, 2013, in forum post "A small change in word strength".