Fault collapsing
Beginner
Removing duplicate pretend flaws that any test would catch together, so the list is shorter.
Novice
Merging faults that are equivalent, meaning every test that catches one also catches the other, so the software only has to handle one fault from each group.
Expert
Equivalence collapsing keeps one fault per class; dominance collapsing goes further for test generation. Coverage on collapsed and uncollapsed lists differs, so say which one a number refers to.