Choose sample size for a mean interval precision target
Solve the margin inequality, round sample size upward and distinguish total width from half-width.
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 zσ/√n.
- Solve a positive inequality and interpret interval width.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
How should a bound n ≥ 138.2976 be converted to sample size?
What you will learn
- Find a minimum independent sample size.
- Check the preceding integer fails the target.
Solve a margin inequality
With known σ = 12 and a 95% target margin at most two, require 1.960 × 12/√n ≤ 2. Squaring positive quantities gives n ≥ 138.2976, so the minimum integer is 139. Rounding to the nearest integer would wrongly select 138.
At n = 138 the margin is approximately 2.0022, while n = 139 gives approximately 1.9949. Check the original margin expression to confirm the first valid integer.
Width is twice the margin
For a target margin at most one, n ≥ 553.1904, so use 554. Halving the margin requires roughly four times as many independent observations at the same confidence and population SD.
A total interval width at most four means margin at most two, giving 139 again. This design controls the formula’s width under its model; it does not guarantee unbiased sampling or repair a wrong population model.
Pause and explain
What margin corresponds to total interval width four?
Worked example
Find minimum n for 95% margin at most two with σ = 12.
Show the worked solution
- Require 23.52/√n ≤ 2.
- Obtain n ≥ 138.2976.
- Round upward to 139 and check 138 fails.
Answer Minimum n = 139.
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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 minimum independent n for σ = 12 and 95% margin at most two.
Give me a hint
Solve the positive margin inequality and take the ceiling.
Compare my reasoning
- n ≥ (1.960 × 12/2)² = 138.2976.
- Minimum integer is 139.
- At 138 the margin exceeds two, so it fails.
n = 139; 138 fails.
Look for these in your work
- Round upward.
- Check the original target.
Find minimum n if the target margin is one instead.
Give me a hint
Keep confidence and population SD unchanged.
Compare my reasoning
- n ≥ (1.960 × 12)² = 553.1904.
- Round upward to 554.
- The smaller margin needs about four times the independent sample.
n = 554.
Look for these in your work
- Square the margin ratio.
- Use the integer ceiling.
Find n for total width at most four, with the same confidence and σ.
Give me a hint
Translate total width to half-width before calculation.
Compare my reasoning
- Half-width is two.
- The same bound n ≥ 138.2976 applies.
- The minimum remains 139.
n = 139; use margin two, not four.
Look for these in your work
- Interpret the target correctly.
- Avoid confusing width and margin.
Common mistakes
- Round n upward.
- Total width is twice margin.
- Count independent observations, not duplicate records.
What can you explain now?
Does width four mean margin four?
Compare with the explanation
No; margin is two.
The interval has a lower and an upper half.
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Make it stick
Solve the half-width target and round n up; check the preceding integer.
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.