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difficulty - estimation of element difficulty in SuperMemo. Two measures can be used to express difficulty: (1) current estimation of element's absolute difficulty is represented by A-Factor (this measure determines the next inter-repetition interval for items), and (2) heuristic measure of difficulty based on the history of repetitions and displayed in the element data window as Difficulty (this measure can be used to sort collections that are to be used by other users; starting with easy items first). An item that is initially difficult (low A-Factor) can become easy to recall once a strong memory imprint is formed. Such an item may quickly reach a relatively high value of A-Factor that reflects current recall difficulty. On the other hand, the Difficulty parameter takes into account, apart from A-Factor, such parameters as the number of lapses, current interval, etc. If an item reaches a long interval on poor grades, it will have a low A-Factor but may have a high Difficulty. At the same time, an item forgotten many times may suddenly reach a high A-Factor while its Difficulty still remains high. Difficulty is expressed as percentage with 0% standing for easiest possible items, and 100% for the most difficult items. A-Factors may be considered as the difficulty from the point of view of SuperMemo Algorithm, while Difficulty is closer to expressing what the user views as the difficulty of learning a given element (to see components of Difficulty[2] press Ctrl+Shift+P and click Difficulty)