Calculate Poisson error probabilities at specified rates
Evaluate a fixed count boundary under different expected counts while preserving exposure.
Section 6.5 · Hypothesis tests
Exam scope and assessment notes
Probability & Statistics 2 (Paper 6, A Level) · Mathematics 9709, 2026–2027, section 6.5. Paper 6 builds on Paper 5 and assumes Paper 3 calculus. Specify population hypotheses, significance level, test statistic and tail direction before inspecting the result. Calculate tail probabilities under H₀, including the observed discrete value. A p-value is not the probability that H₀ is true. Reject H₀ when the specified tail probability is at most the stated significance threshold; otherwise do not reject. The syllabus term acceptance region means the non-rejection region, not proof of H₀. For two-sided discrete examples here, use an explicitly stated equal-tail allocation; asymmetric discrete models admit different conventions, so do not treat twice the smaller tail as a universal definition. Integer critical regions can have actual Type I probability below the nominal level. Normal approximations need a suitable model and continuity correction for counts; check exact probabilities when approximation could change a close decision. Mean tests use a normal population with known variance or an adequate large independent sample with estimated variance. Small samples with unknown variance do not automatically justify the z method. Type I means rejecting a true null; Type II means not rejecting a false null at a specified alternative. Calculate error probabilities using the fixed decision region under the relevant parameter, not by changing the test after seeing data. Power is one minus Type II probability at that alternative. Quoted normal critical values and table probabilities have rounding limits. Interpret evidence in context without claiming certainty, causation or practical importance. Written practice is self-checked; cumulative assessment and human subject-teacher review remain pending.
Before you begin
- Use Poisson cumulative probabilities.
- Identify a complementary integer event.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is non-rejection when the test rejects X ≥ 9?
What you will learn
- Calculate Poisson Type I and Type II probabilities.
- Keep test rule and exposure fixed.
Two models, one decision boundary
The upper test of an hourly mean four rejects for X ≥ 9. Actual Type I probability is P₄(X ≥ 9) ≈ 0.0213634345. At true hourly mean eight, the non-rejection event remains X ≤ 8, giving β = P₈(X ≤ 8) ≈ 0.5925473414.
The corresponding power at mean eight is about 0.4074526586. It is not 0.9786365655: one minus the null Type I probability describes null non-rejection, not sensitivity under mean eight.
Compare specified alternatives
At true hourly mean twelve, β = P₁₂(X ≤ 8) ≈ 0.1550277818, so power is about 0.8449722182. Keep one-hour exposure for both calculations; doubling the duration would change the count model and the test’s design.
A single count does not reveal whether an error occurred because the true rate is usually unknown. These probabilities describe the specified procedure in repeated samples at stated rates.
Pause and explain
Which distribution is used for β at true hourly mean eight?
Worked example
Find β at hourly mean eight for rejection X ≥ 9.
Show the worked solution
- Non-rejection is X ≤ 8.
- Use Po(8) at the stated alternative.
- β ≈ 0.5925473414; power ≈ 0.4074526586.
Answer β ≈ 0.5925473414.
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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.
Find actual Type I probability for the Po(4) rule X ≥ 9.
Give me a hint
Evaluate the fixed rejection event under the null.
Compare my reasoning
- Use Po(4).
- Complement the cumulative sum through eight.
- Actual Type I probability ≈ 0.0213634345.
0.0213634345.
Look for these in your work
- Use null mean four.
- Keep the inclusive boundary.
Find β and power at true mean eight with the same hourly rule.
Give me a hint
Calculate P(X ≤ 8) under Po(8).
Compare my reasoning
- β ≈ 0.5925473414.
- Power = 1 − β.
- Power ≈ 0.4074526586.
β 0.5925473414; power 0.4074526586.
Look for these in your work
- Use the alternative rate.
- Use the non-rejection region.
Find β and power at true hourly mean twelve.
Give me a hint
Keep the one-hour rule and change only the true distribution.
Compare my reasoning
- Use Po(12) and event X ≤ 8.
- β ≈ 0.1550277818.
- Power ≈ 0.8449722182.
β 0.1550277818; power 0.8449722182.
Look for these in your work
- Keep exposure unchanged.
- Compare specified alternatives.
Common mistakes
- Complement ≥ 9 to ≤ 8.
- Keep observation duration fixed.
- Do not replace power by 1 − α.
What can you explain now?
Does 1 − α equal power at a different true rate?
Compare with the explanation
Not generally.
The probabilities use different rate distributions.
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Make it stick
A Poisson error calculation changes the true mean while preserving the original count rule.
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.