Definition
Certain event
A certain event is guaranteed by the stated chance setup and has probability 1.
What it means
A certain event must happen when the chance experiment follows its stated rules. Every possible result makes the event true. Change the setup, and what is certain may change too.
Remember it
Every possible path lands inside a certain event.
Example
A bag contains only blue and green counters. The event “the counter chosen is blue or green” is certain because every counter in the bag makes the event happen.
Watch for
An event that is very likely is not certain if even one possible outcome would make it fail.
Common mix-up
“Certain” means guaranteed under the stated conditions, not simply expected or very likely.
Exact meaning
A certain event contains every possible outcome in the stated sample space, so its probability is 1. Certainty is always relative to the conditions of the probability model.