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Collecting data and sampling

Qualitative (categorical) and quantitative (discrete and continuous); primary and secondary data.

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Types of data

Qualitative (categorical) and quantitative (discrete and continuous); primary and secondary data.

Populations and samples

A census surveys the whole population; a sample surveys part of it. Sampling frames and bias.

Sampling methods

  • Simple random sampling (random numbers), systematic sampling (every kth member), stratified sampling (proportional to group sizes), quota sampling, cluster sampling.

Worked example: A school has 360 boys and 240 girls. A stratified sample of 50 is required. How many girls should be chosen?

240 ÷ 600 × 50 = 20 girls

Questionnaires and surveys

Designing clear, unbiased questions; open and closed questions; pilot surveys; methods of contact (face-to-face, telephone, online).

Experiments and observation

Control of variables; recording results accurately.

Exam tip: In stratified sampling, check that your group sizes add up to the total sample size.

Check yourself

  1. What is a sampling frame?
  2. Describe systematic sampling.
  3. Give one reason for a pilot survey.
Show answers
  1. A list of all the members of the population.
  2. Choose a random starting point, then select every kth member of the list.
  3. To test the questions for clarity and identify problems before the main survey.
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