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estimation of item's difficulty in SuperMemo. The three main measures of difficulty:
  1. current estimation of item's difficulty computed by Algorithm SM-18. This number ranges from 0 (for easy items) to 1 (for difficult items). It is displayed as Diff in the Element data window (see: details)
  2. current estimation of item's absolute difficulty estimated by Algorithm SM-15. This number ranges from 1.2 (for difficult items) to 6.9 (for easy items). It is represented by A-Factor and displayed as A-Factor in the Element data window. This value reflects bounded stabilization for first review executed at optimum interval
  3. heuristic measure of difficulty based on selected item data, and displayed in the Element data window as Old (this measure can be used to sort collections that are to be used by other users; starting with easy items first)