Distinguish independence from mutual exclusion
Test whether knowing one event changes the other by comparing intersection probability with the product of marginals.
Exam scope and assessment notes
Probability & Statistics 1 (Paper 5, AS & A Level) · Mathematics 9709, 2026–2027, section 5.3. Define the sample space and state its probability assumptions before counting. Fair independent dice give equiprobable ordered pairs; their sums are not equiprobable. Random selection means each specified individual is equally likely under the stated sampling model. Without replacement, later branch probabilities change; with replacement and independent mixing, they remain the same. A′ is a complement, A ∩ B means both, A ∪ B means either or both, and P(A|B) means A given B with P(B) > 0. Mutually exclusive and independent are different properties. The explicit general addition formula is not required by this section; explain overlap through disjoint regions. Conditional probabilities may use P(A|B) = P(A ∩ B)/P(B). Numerical models describe the stipulated experiment, not automatically a real population. Written practice is self-checked; human subject-teacher review remains pending.
Before you begin
- Calculate union and intersection probabilities.
- Multiply fractions and read event membership.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which calculation tests independence?
What you will learn
- Test independence using P(A ∩ B) = P(A)P(B).
- Explain why positive-probability mutually exclusive events are dependent.
Independence is a probability equality
Events A and B are independent when P(A ∩ B) = P(A)P(B). On one fair die, let A be even and B be a multiple of 3. Then P(A) = 1/2, P(B) = 1/3 and their intersection is {6}, with probability 1/6. Since (1/2)(1/3) = 1/6, they are independent even though both refer to the same die.
Independence does not mean the events cannot occur together or must involve separate physical experiments. A stipulated model can make two properties of one outcome independent. Test the equality from supplied probabilities or an equiprobable sample space instead of judging from wording alone.
Exclusive positive events cannot be independent
Mutually exclusive events have no shared outcome, so P(A ∩ B) = 0. If both marginal probabilities are positive, their product is positive and they cannot be independent. For A = {1,2} and B = {3,4} on a fair die, the intersection is empty but P(A)P(B) = 1/9.
In a one-hundred-outcome equiprobable table, A occurs 40 times, B 50 times and both 20 times. Joint probability 0.20 equals 0.40 × 0.50, so the model’s events are independent. A finite observational table can show the equality for its recorded distribution; it does not by itself prove a stable population mechanism or causal relation. A zero-probability event is an exceptional case: the simple positive-probability argument above does not apply to it.
Pause and explain
Fair die: even and multiple of 3 have joint probability 1/6. Are they independent?
Worked example
On one fair die, A is even and B is a multiple of 3. Test independence and mutual exclusion.
Show the worked solution
- P(A) = 3/6 = 1/2 and P(B) = 2/6 = 1/3.
- The joint event is {6}, so P(A ∩ B) = 1/6 = P(A)P(B).
- They are independent but not mutually exclusive, since 6 satisfies both.
Answer Independent; not mutually exclusive.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
For one fair die, test independence of A = even and B = multiple of 3. Also state whether they are mutually exclusive.
Give me a hint
List their intersection before comparing it with the marginal product.
Compare my reasoning
- The marginals are 1/2 and 1/3.
- The intersection {6} has probability 1/6, equal to their product.
- They are independent, but not mutually exclusive because the intersection is possible.
Independent; not mutually exclusive.
Look for these in your work
- Show the numerical equality.
- Use separate tests for the two properties.
In a model of 100 equally likely outcomes, A occurs 40 times, B 50 times and both 20 times. Test independence.
Give me a hint
Convert the three counts to probabilities with the same denominator.
Compare my reasoning
- P(A) = 0.40, P(B) = 0.50 and P(A ∩ B) = 0.20.
- The product 0.40 × 0.50 equals 0.20.
- The events are independent in this specified finite model.
Independent in the model: 0.20 = 0.40 × 0.50.
Look for these in your work
- Use consistent total counts.
- Limit the conclusion to the stated distribution.
Mutually exclusive events have P(A) = 0.30 and P(B) = 0.40. Test independence and explain what observing A tells you about B.
Give me a hint
Mutual exclusion makes their intersection impossible.
Compare my reasoning
- The joint probability is zero.
- The marginal product is 0.30 × 0.40 = 0.12, which differs from zero.
- They are dependent; observing A rules out B, changing its probability from 0.40 to zero.
Dependent: 0 ≠ 0.12; observing A rules out B.
Look for these in your work
- Use the positive marginal probabilities.
- Explain the information change as well as the arithmetic.
Common mistakes
- Independence allows a shared outcome.
- Separate descriptions do not prove independence.
- Distinguish exact model equality from population evidence.
What can you explain now?
Can two mutually exclusive events of positive probability be independent?
Compare with the explanation
No; their joint probability is zero but the product of their marginals is positive.
The independence equality fails under those assumptions.
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Make it stick
Exclusion asks whether both can happen; independence asks whether the joint probability equals the marginal product.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.