M4.7.1 For the adult
Samples and bias
Ravi asks 20 people leaving the sports hall if they exercise every day. Do his answers tell us about the whole town?
What this chapter teaches
A sample is biased when some members of the population are more likely to be chosen. A random sample gives every member the same chance. A stratified sample takes each group in proportion to its size: group ÷ whole × sample size.
Three questions to ask
There is no one right wording. Listen for the idea.
- Why does asking 500 people at one place not make a fair sample of a town?
- How could you use random numbers to choose 5 plants from a bed of 40?
- A club has 60 members, 36 of them walkers. How many walkers go in a sample of 10, and why?
After the chapter, the learner can
- Explain how a sample can be biased
- Take a random sample using random numbers
- Choose a sample that matches the groups in a population
What they copy into their notebook
A sample is biased if some members are more likely to be chosen. A random sample gives every member the same chance. A stratified sample takes group ÷ whole × sample size from each group.