Select a random sample using labelled random numbers
Use a complete numbered frame and an explicit rule for invalid labels and repeats.
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
- Number a finite list consistently.
- Distinguish sampling with and without replacement.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which two-digit labels are valid for a frame numbered 01–50?
What you will learn
- Use random numbers for elementary selection.
- Explain rejection of invalid or repeated labels.
State a selection rule
For a frame labelled 01 through 50, read uniformly generated two-digit random numbers. For a sample without replacement, retain labels 01–50 and reject 00, 51–99 and labels already selected. Continue until the required number of distinct people is reached.
The illustrative stream 07,52,00,07,41,18,50,41,03 selects 07,41,18,50,03 for five people. Rejections prevent impossible labels and duplicate inclusion; they do not mean replacing a rejected label with the next convenient person.
Random numbers do not define the population
Every valid frame member needs the intended chance of selection. A deliberately chosen starting label or stopping when a preferred answer appears destroys the stated random mechanism.
The stream is a worked illustration, not evidence that a real survey was random. Keep the frame current and handle non-response separately. Particular named sampling methods are not required by this section.
Pause and explain
Which five labels are retained from the supplied stream without replacement?
Worked example
Select five distinct labels using the supplied stream for 01–50.
Show the worked solution
- Keep 07; reject 52,00 and repeated 07.
- Keep 41,18,50; reject repeated 41.
- Keep 03 to finish five distinct selections.
Answer 07,41,18,50,03.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
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.
Select five distinct labels from 07,52,00,07,41,18,50,41,03 for frame 01–50.
Give me a hint
Reject invalid labels and repeats, in stream order.
Compare my reasoning
- First accepted label is 07.
- Next accepted labels are 41,18,50.
- Reject repeated 41 and accept 03.
07,41,18,50,03.
Look for these in your work
- Keep the original stream order.
- Select distinct valid labels.
For frame 01–20, select three without replacement from 00,04,21,04,19,08.
Give me a hint
Apply the same validity and duplicate rules to the new frame.
Compare my reasoning
- Reject 00 and 21.
- Accept 04 and reject its repeat.
- Accept 19 and 08.
04,19,08.
Look for these in your work
- Use the correct label range.
- Reject repeated selection.
Would the repeat-rejection rule be appropriate if the experiment explicitly samples with replacement?
Give me a hint
Separate the selection design from the label-validity rule.
Compare my reasoning
- Labels outside the frame still remain invalid.
- With replacement, repeated valid selections are allowed.
- Rejecting repeats would change that stipulated mechanism.
No; valid repeats are allowed with replacement, while invalid labels are still rejected.
Look for these in your work
- State the sampling mechanism.
- Do not silently change the design.
Common mistakes
- Reject labels outside the frame.
- Reject repeats only for without-replacement selection.
- Do not substitute a convenient person.
What can you explain now?
Why reject the second 07?
Compare with the explanation
The sample is without replacement.
That person is already included.
After trying it yourself, choose your next review. This is your self-assessment.
Your review choice appears on Today. Sign in to sync it across devices.
Make it stick
Random-number selection needs valid labels and a replacement rule stated in advance.
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.