Complementary Events
Complementary events are a subgroup of mutually exclusive events. Mutually exclusive events (including complementary events) can’t both happen; but when two events are complementary then one or the other must happen.
For example, the events “you get an A in this course” and “you get a B in this course” are mutually exclusive but not complementary; the events “you get a C or better” and “you get a D or worse” are mutually exclusive and complementary.
If two events are complementary, their probabilities must add to 1 (certainty) and each of them is equivalent to not the other.
Learn more about complementary events by reading the explanation below.
Practice: Answer the following exercises about complementary events.