Leech filter dialog box is used to define the criteria for classifying items as leeches (a leech is an intractable item that causes particular difficulties in learning). The following three criteria may be used:
* maximum E-factor (the lower the E-factor, the more difficult the item)
* minimum number of lapses, i.e. the minimum number of times the classified item has been forgotten in the process of learning (the greater the number of lapses, the more difficult the item)
* maximum interval (literal “All” can be used to ignore this criterion). Note, that it may be useful to exclude long-interval items from among the leeches, as the difficulties with remembering such items might have already been overcome.