Revising sampling & bias the night before an exam? These are condensed revision notes — the facts, formulas and mistakes examiners actually mark, with no long explanations. Prefer a full walkthrough with step-by-step reasoning instead? Search “Sampling & bias GCSE” on the site for the complete guide.
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Key facts to remember
- 1A random sample gives every member of the population an equal chance of being selected.
- 2A biased sample over- or under-represents certain groups.
- 3Larger samples give more reliable results.
- 4Stratified sampling selects from each subgroup in proportion to its size.
- 5Common sources of bias: asking leading questions, sampling only convenient groups, timing of survey.
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Formulas
Sample size from group = (group size / population) × total sample size
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Worked examples
Example 1A school has 400 Year 10 and 600 Year 11 students. A stratified sample of 50 is taken. How many should be from Year 10?
Working
- Year 10 proportion = 400/1000 = 2/5
- Year 10 in sample = (2/5) × 50 = 20
Answer20 students from Year 10
Example 2A survey asks "Don't you agree our school canteen food is terrible?" Identify the bias.
Working
- The question is leading — it suggests a negative view
- Respondents are more likely to agree due to the phrasing
- This introduces response bias
AnswerThe question is biased as it leads respondents towards a negative answer.
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Common mistakes
✗Confusing a large sample with an unbiased sample — a large biased sample is still biased.
✗Not rounding stratified sample sizes to whole numbers.
✗Failing to identify how a question or method introduces bias.
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Exam tips
✓For bias questions, explain specifically how the method excludes certain groups or influences responses.
✓For stratified sampling, always calculate the proportion first, then multiply by the total sample size.
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