Calculate binomial error probabilities for a fixed test
Evaluate the same critical region under the null and its complement under specified alternatives.
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 exact binomial tails.
- Distinguish null and alternative parameters.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is the non-rejection region for X ≥ 8?
What you will learn
- Calculate exact binomial Type I and Type II probabilities.
- Interpret power at different alternatives.
Keep the region fixed
The 5% increase test of p = 0.2 with n = 20 rejects for X ≥ 8. Its actual Type I probability is P₀.₂(X ≥ 8) ≈ 0.0321426631. The non-rejection region is X ≤ 7.
At true p = 0.4, Type II probability is P₀.₄(X ≤ 7) ≈ 0.4158929376. Power is approximately 0.5841070624. Do not find a new 5% region under p = 0.4: the procedure is still the original test.
The alternative determines β
At true p = 0.5, the same non-rejection probability is P₀.₅(X ≤ 7) ≈ 0.1315879822, and power is approximately 0.8684120178. A larger upward change is more readily detected by this fixed upper-tail rule.
The Type II result is not a probability that H₀ is false after one observation. It is a repeated-sampling non-rejection probability when the specified alternative is true.
Pause and explain
Which model calculates β when true p = 0.4?
Worked example
Find β at p = 0.4 for the rule X ≥ 8.
Show the worked solution
- The non-rejection event is X ≤ 7.
- Evaluate that event under B(20,0.4).
- β ≈ 0.4158929376; power ≈ 0.5841070624.
Answer β ≈ 0.4158929376.
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Test this topic →From guided practice to a new situation
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 rejecting X ≥ 8 under H₀: p = 0.2, n = 20.
Give me a hint
Evaluate rejection under the null model.
Compare my reasoning
- Use B(20,0.2).
- Sum the upper tail from eight.
- Actual Type I probability ≈ 0.0321426631.
0.0321426631.
Look for these in your work
- Use the true null.
- Keep the rejection event.
Find β and power at true p = 0.4 for that fixed rule.
Give me a hint
Use B(20,0.4) and the non-rejection event X ≤ 7.
Compare my reasoning
- β = P₀.₄(X ≤ 7) ≈ 0.4158929376.
- Power = 1 − β.
- Power ≈ 0.5841070624.
β 0.4158929376; power 0.5841070624.
Look for these in your work
- Use the alternative model.
- Retain the fixed boundary.
Compare β and power at true p = 0.5 using the same rule.
Give me a hint
Evaluate the same non-rejection event under B(20,0.5).
Compare my reasoning
- β ≈ 0.1315879822.
- Power ≈ 0.8684120178.
- This larger upward alternative is more readily detected by the fixed test.
β 0.1315879822; power 0.8684120178.
Look for these in your work
- Change the model, not the rule.
- Compare at stated alternatives.
Common mistakes
- The rule stays fixed under alternatives.
- Use the complement for Type II.
- β is not a posterior probability.
What can you explain now?
When calculating β, should the critical threshold be reselected under the alternative?
Compare with the explanation
No.
That would describe a different test.
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Make it stick
Calculate Type I in the null rejection region and Type II in the alternative non-rejection region.
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