Strengthened skills

Strength is a measure Duolingo uses 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].

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 strengthEdit


The Words section showing the strengths of individual words.

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] Strengthen skills sessions will choose some of the weakest words for practice.[3] The strengths of the learned words can be viewed in the Words section (in courses where it is available).

Skill strengthEdit

Decayed Skill

A skill with low strength.

Skill strength is a function of the strengths of the words taught within the skill.[4] It is indicated on the semicircular meter on the left side of a completed skill. Note that a skill being at "full strength" does not necessarily mean that all words within the skill are at full strength[4].

Course strengthEdit


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 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 decayEdit

The decay rate of a word depends 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 decayEdit

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 rateEdit

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 alsoEdit


  1. 1.0 1.1 1.2 1.3 1.4
  2. "A New Duolingo for iOS 7". Retrieved 2014-07-11.
  3. 3.0 3.1 3.2 3.3 3.4 3.5 "Notice some changes in word strength?".
  4. 4.0 4.1 4.2 Comment by Burr Settles (tatou) in forum post "German Tree Bugged". Retrieved 2015-03-22.
  5. 5.0 5.1 Comment by Burr Settles (tatou) in forum post "What Language Acquisition Theory(s) Is Used in Duolingo?".
  6. 6.0 6.1 Comment by Burr Settles (tatou) in forum post 'Does "lesson practice" function has to be close to useless?'.
  7. Comment by Burr Settles (tatou) in forum post "A small change in word strength".