Interpret confidence as repeated-sampling coverage
Explain what a confidence procedure covers and avoid probability claims about a fixed realised interval or individual data.
Exam scope and assessment notes
Probability & Statistics 2 (Paper 6, A Level) · Mathematics 9709, 2026–2027, section 6.4. Paper 6 builds on Paper 5 and assumes Paper 3 calculus. Distinguish population parameters, sample statistics and repeated-sample random variables. Random selection needs a relevant frame and does not automatically remove undercoverage or non-response. Particular named sampling methods are not required. Sample-mean formulae here assume independent observations from one population with finite variance: E(X̄) = μ, Var(X̄) = σ²/n and SE = σ/√n. A mean of independent normal observations is normal; the CLT gives an approximate normal mean for an adequate large independent sample, not normal individual data. No universal sample-size cutoff guarantees that approximation. Unbiased sample variance uses n − 1, with n > 1. Mean intervals use the normal model with known variance or a large-sample approximation; a small sample with unknown variance does not justify the z method taught here. Proportion intervals use the large-sample normal approximation with adequate success and failure counts; extreme or sparse counts may invalidate it. Quoted two-sided critical values are 1.645 for 90%, 1.960 for 95% and 2.576 for 99%; retain other working unrounded. Confidence describes repeated-sampling coverage, not posterior probability for a fixed realised interval, individual observations or a guarantee. A narrower interval does not repair a biased sample. Hypothesis testing belongs to section 6.5. Written practice is self-checked; cumulative assessment and human subject-teacher review remain pending.
Before you begin
- Construct a mean interval.
- Distinguish a fixed parameter from a random statistic.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which changes over repeated samples in this confidence interpretation?
What you will learn
- Interpret repeated-sample confidence coverage.
- Distinguish estimation from a prediction interval.
The procedure varies; the parameter stays fixed
Before collecting data, the random interval varies with the sample. A correctly calibrated 95% procedure contains the fixed population parameter in approximately 95% of repeated samples under its assumptions. With rounded normal critical values the intended coverage is approximate at the quoted precision.
After observing data, both interval endpoints are fixed numbers. The interval either contains the fixed parameter or it does not. This frequentist statement does not assign a 95% posterior probability to μ being inside that one realised interval.
A mean interval is not an individual range
An interval for μ describes uncertainty about the population mean. It does not claim that 95% of individual travel times lie in that interval, nor that the next sample mean must lie there. Population spread and parameter uncertainty are different quantities.
In 1000 repetitions of a calibrated 95% procedure, the expected number covering μ is about 950; the actual number fluctuates. A picture of several illustrative intervals is not a simulation proving calibration, and bias or invalid sampling assumptions can change coverage.
Pause and explain
Does a 95% mean confidence interval contain 95% of individual observations?
Worked example
Interpret a 95% confidence procedure for the population mean.
Show the worked solution
- Repeat the same sampling and interval construction many times.
- The parameter remains fixed while the intervals vary.
- About 95% of such intervals contain the parameter under the assumptions.
Answer Repeated-interval coverage, not a posterior probability for one fixed interval.
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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.
What is the expected coverage count among 1000 repetitions of a calibrated 95% procedure?
Give me a hint
Use probability times number of repetitions.
Compare my reasoning
- Each interval has intended coverage probability about 0.95.
- Expected covering count is 1000 × 0.95.
- Actual count is random, not guaranteed to equal that expectation.
About 950 expected; not exactly 950 guaranteed.
Look for these in your work
- State repeated-sampling meaning.
- Distinguish expectation from a realised count.
Explain why a 95% interval for μ does not describe 95% of individual values.
Give me a hint
Identify the parameter targeted by the formula.
Compare my reasoning
- The interval targets population mean μ.
- Its scale uses the mean’s SE rather than population SD.
- Individual observations retain their own spread.
It estimates the mean; it is not an individual prediction interval.
Look for these in your work
- State the target.
- Separate SE from individual SD.
Assess the claim: after calculating [46.08,53.92], there is a 95% posterior probability that fixed μ lies inside.
Give me a hint
Use the repeated-sampling meaning actually supplied by this procedure.
Compare my reasoning
- The observed endpoints are now fixed.
- Fixed μ is either inside or outside.
- No prior or posterior model was supplied; frequentist coverage is a procedure property.
Unsupported posterior claim; the 95% refers to repeated-interval coverage.
Look for these in your work
- Keep the parameter fixed.
- Do not add a posterior interpretation without its model.
Common mistakes
- A realised frequentist interval does not make μ random.
- Confidence does not describe individual observation coverage.
- Expected coverage counts are not guaranteed realised counts.
What can you explain now?
Must exactly 95 out of 100 intervals cover μ?
Compare with the explanation
No.
Coverage counts themselves fluctuate.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Confidence is coverage of a repeated procedure under assumptions, not a guarantee about one realised interval.
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.