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9609 · 3.2.3

Sampling — practice questions

Practice and worked examples for 9609 Sampling. Short previews only — attempt the full question in MarkScheme against the official scheme.

Worked example 1

A skincare brand targets women aged 25–40 nationwide. A student surveys 50 classmates. Evaluate the sampling.

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Highly biased / unrepresentative.

Problems: Wrong age profile (likely 16–18); geographic cluster (one school); convenience sample not random or stratified.

Improvement: Stratified random sample of women 25–40 across regions; online panel or random phone/email from customer database; minimum 200+ for basic reliability.

Invalid sample → wrong product/pricing decisions — wasted marketing spend.

Worked example 2

A streaming service has a total of 80,000 subscribers in a country. They want to conduct a survey on a new pricing plan using a sample of 1,500 subscribers. The subscriber base is segmented as follows:

  • Under 25: 24,000 subscribers
  • 25-44: 40,000 subscribers
  • 45 and over: 16,000 subscribers

Calculate the number of subscribers that should be surveyed from each age group using a stratified sampling method.

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The goal is to create a sample where the proportion of each age group matches the proportion in the total subscriber population.

Formula: Sample for stratum = (Size of stratum / Total population) × Total sample size

Step 1: Calculate the sample size for the 'Under 25' stratum. Proportion = 24,000 / 80,000 = 0.30 (or 30%) Sample size = 0.30 × 1,500 = 450 subscribers

Step 2: Calculate the sample size for the '25-44' stratum. Proportion = 40,000 / 80,000 = 0.50 (or 50%) Sample size = 0.50 × 1,500 = 750 subscribers

Step 3: Calculate the sample size for the '45 and over' stratum. Proportion = 16,000 / 80,000 = 0.20 (or 20%) Sample size = 0.20 × 1,500 = 300 subscribers

Final Answer & Verification: The research team should survey:

  • 450 subscribers from the 'Under 25' group
  • 750 subscribers from the '25-44' group
  • 300 subscribers from the '45 and over' group

Total sample = 450 + 750 + 300 = 1,500. This matches the required total sample size.

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