Disjoint vs. independent: In parts (a) and (b), identify whether the events are
a. disjoint,
b. independent,
c .or neither (events cannot be both disjoint and independent).
a) You and a randomly selected student from your class both earn A’s in this course.
a. neither
b. disjoint
C. independent X
b) You and your class partner both earn Alhin this course. disjoint independent neither X
C) if two events can occur at the same time, they must be independent
The correct answer and explanation is :
Alright, let’s go through this carefully:
(a) You and a randomly selected student from your class both earn A’s in this course.
- Answer: C. Independent
Explanation:
You getting an A and another student getting an A are not disjoint — they can both happen together. Disjoint events cannot happen at the same time, but here, both earning an A can definitely occur simultaneously.
Also, your performance is not necessarily influenced by the other student’s performance (assuming no group grading), so the events can be considered independent — one event does not affect the probability of the other.
(b) You and your class partner both earn A’s in this course.
- Answer: C. Independent (assuming individual work)
Explanation:
Same logic applies: both you and your partner earning A’s can happen together — so the events are not disjoint.
If you and your partner work separately and your grades are assigned independently, the events can be considered independent.
However, if you were graded as a team (like on group projects), the events might not be independent, but the typical assumption is that you are graded separately, so independent is correct.
(c) If two events can occur at the same time, they must be independent.
- Answer: False
Explanation:
Two events happening at the same time just means they are not disjoint, but it does not mean they are independent. Independence is about whether the occurrence of one event affects the probability of the other.
For example, consider these events:
- Event A: It rains today.
- Event B: The ground gets wet.
These two events can happen together, but they are clearly not independent — rain makes the ground wet! So just because two events can occur together, it does not guarantee independence.
Full 300-Word Explanation
In probability theory, disjoint (also called mutually exclusive) events are events that cannot happen at the same time. If one event occurs, the other cannot. Independent events, however, are events where the occurrence of one has no effect on the probability of the other occurring. Importantly, events cannot be both disjoint and independent unless one of them has zero probability.
In part (a), the events — you and a randomly selected classmate both earning A’s — are independent. Your grade does not influence theirs and vice versa (assuming normal grading). Since both events can happen together (both can earn A’s), they are not disjoint.
In part (b), the situation is similar. You and your partner both earning A’s is independent if your grades are assigned individually. They are not disjoint because it’s possible for both to earn A’s. However, if you were graded together, then the events might be dependent. Based on the wording, we assume independence.
For part (c), the statement is false. Two events occurring at the same time does not mean they are independent. Independence is a special relationship where knowing that one event occurred gives no information about the other. Just because events can happen together (i.e., they are not disjoint) does not guarantee they are independent. For example, raining and the ground being wet often happen together, but they are dependent, not independent.
Understanding the difference between disjoint and independent is crucial in probability because it affects how we compute the probabilities of combined events.