Distinguish populations samples and sampling frames
Define the population you want to describe, the observations you collect and the list from which you select.
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
- Interpret a statistical question.
- Distinguish an observation from a summary.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which group is the population in this travel-time question?
What you will learn
- Distinguish a population from its sample.
- Identify undercoverage in a sampling frame.
Define the target before selecting
A school wants the mean travel time of all 800 enrolled students. Those 800 form the target population. The 80 students selected for measurement form the sample. The population mean μ is fixed but unknown; the sample mean x̄ changes when a different sample is drawn.
The sampling frame is the list used to select people. A current enrolment list can cover the intended population. A list of library members may exclude students who do not use the library, even if selection from that list is random.
Randomness is necessary but needs a relevant frame
Random selection helps avoid choosing people because of their likely answers and supports probability-based inference. It cannot include people omitted from the frame or force selected people to respond.
A survey of volunteers can be easier to collect, but willingness to respond may be related to travel time. Describe the possible selection mechanism instead of assuming a large response count makes the sample representative.
Pause and explain
Can random selection repair omissions from its sampling frame?
Worked example
Identify population, sample and frame for 80 random selections from an 800-student enrolment list.
Show the worked solution
- Target population: all 800 enrolled students.
- Sample: the 80 selected observations.
- Frame: the enrolment list used for selection.
Answer Population 800; sample 80; frame enrolment list.
Try a topic test
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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.
For 80 selected from an 800-student enrolment list, identify population, sample and frame.
Give me a hint
Separate the target group, observed subset and selection list.
Compare my reasoning
- Population is all 800 enrolled students.
- Sample is the 80 selected students.
- Frame is the enrolment list.
Population 800; sample 80; frame enrolment list.
Look for these in your work
- State the target group.
- Keep frame and sample distinct.
A random survey uses only the library-member list. Explain one weakness for the school-wide travel-time question.
Give me a hint
Ask who cannot appear in the sample.
Compare my reasoning
- Non-members are absent from the frame.
- They have zero selection chance through this list.
- Their travel times may differ, creating undercoverage.
Undercoverage of non-library members; random selection within the list does not fix it.
Look for these in your work
- Identify the omitted group.
- Connect omission to the target question.
A large voluntary online response replaces random selection. Does size alone justify school-wide inference?
Give me a hint
Consider whether response willingness can be related to the answer.
Compare my reasoning
- Students choose whether to respond.
- That choice may correlate with travel time.
- A large number reduces neither that selection mechanism nor its possible bias automatically.
No; voluntary response may be biased despite a large sample.
Look for these in your work
- Distinguish precision from selection quality.
- Avoid an unsupported representative claim.
Common mistakes
- The sample is not the full target population.
- A random frame sample can still have undercoverage.
- Define the measurement and target group.
What can you explain now?
Does a random library-member sample represent every enrolled student automatically?
Compare with the explanation
No.
The frame may omit non-members.
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Make it stick
Define population, frame and sample before calculating an estimate.
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.