# Glossary:Difficulty

**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**)