Connect sampling estimates and confidence claims in context
Calculate mean and proportion intervals from one study while keeping their different targets and design limitations clear.
Section 6.4 · Sampling and estimation
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
- Use unbiased summary estimates.
- Construct large-sample mean and proportion intervals.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does the binary threshold interval target?
What you will learn
- Combine estimates with suitable interval methods.
- Report distinct parameter targets and unresolved bias.
Different outcomes have different targets
A stipulated independent relevant sample has n = 100, Σx = 5000 and Σx² = 251584 for a continuous score. It gives x̄ = 50 and s² = 16. Estimated SE is 0.4, giving an approximate 95% mean interval [49.216,50.784] when the large-sample approximation is adequate.
If 60 of those 100 also meet a consistently defined threshold, p̂ = 0.6 and estimated proportion SE is √0.0024. Its approximate 95% interval is [0.5040,0.6960]. This targets a population threshold proportion, not the mean score; the two intervals are not interchangeable.
Interpret the actual design
The mean and binary outcomes can be measured on the same people without making their parameter targets identical. The assumptions concern independent sampling across people; this lesson does not claim that two outcomes on one person are independent.
If the data instead come from volunteers selected for interest in the topic, the nominal formula margins do not measure that selection bias. Report the limitation and avoid claiming population representativeness solely because the computed intervals are narrow.
Pause and explain
Do narrow calculated intervals repair volunteer-selection bias?
Worked example
Find the mean estimate and variance estimate from the continuous score totals.
Show the worked solution
- Mean = 5000/100 = 50.
- Centred squared sum = 251584 − 250000 = 1584.
- Unbiased variance = 1584/99 = 16.
Answer Mean 50; variance 16.
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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 n = 100, Σx = 5000 and Σx² = 251584, find unbiased estimates and a suitable large-sample 95% mean interval.
Give me a hint
Estimate variance before forming the mean’s SE.
Compare my reasoning
- Mean = 50; s² = 1584/99 = 16.
- Estimated SE = 4/10 = 0.4; margin = 0.784.
- Interval endpoints are 49.216 and 50.784.
Mean 50; variance 16; approximate interval [49.216,50.784].
Look for these in your work
- Use n − 1.
- State mean target and approximation.
With 60 threshold successes from the same 100 people, find a suitable approximate 95% proportion interval.
Give me a hint
Use p̂ = 0.6 and the binary SE formula.
Compare my reasoning
- Estimated SE = √(0.6 × 0.4/100).
- Margin ≈ 0.0960200.
- Endpoints ≈ 0.5040 and 0.6960.
Approximately [0.5040,0.6960] for the population threshold proportion.
Look for these in your work
- Use binary variance.
- Distinguish proportion from mean.
The study actually used interested volunteers. What limitation should accompany those nominal intervals?
Give me a hint
The margins describe the stated sampling model, not all data-collection error.
Compare my reasoning
- Volunteer selection may relate to the outcomes.
- That can bias both estimates for the target population.
- The calculated margins do not repair or measure that bias.
Volunteer-selection bias remains; narrow nominal intervals do not establish population representativeness.
Look for these in your work
- Explain the actual selection mechanism.
- Avoid unsupported population claims.
Common mistakes
- Different parameter targets need different formulae.
- Narrow intervals do not establish representative sampling.
- Do not add independence between outcomes without evidence.
What can you explain now?
Does a mean interval estimate the fraction exceeding a threshold?
Compare with the explanation
No.
That fraction is a separate population proportion.
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Make it stick
Estimate the right population target, apply a justified method and report what the design still cannot support.
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.