Final answer:
Events are collectively exhaustive when their union includes all possible outcomes of an experiment, leaving no outcomes unaccounted for.
Step-by-step explanation:
Events are collectively exhaustive if their union covers the entire sample space, meaning that the combination of the events includes every possible outcome from the experiment. In other words, there are no outcomes left uncovered or unaccounted for.
For example, consider an experiment of flipping a fair coin. Let event A be getting heads and event B be getting tails. In this case, events A and B are collectively exhaustive because together they cover all possible outcomes of the experiment (getting either heads or tails).
In sum, events are collectively exhaustive when their union includes all possible outcomes of an experiment, leaving no outcomes unaccounted for.