Collecting data and sampling · Year 12
In Year 12 you learn: capture–recapture.
To learn about a whole population, you usually collect data from only a sample. A good sample is big enough and represents the population fairly. A biased sample gives misleading results, however carefully you analyse it.
Key facts
- Population: everyone or everything you want to find out about. Sample: the part you actually collect data from.
- Stratified sample: sample each group in proportion. Number from a group = (group size)/population × sample size.
- Convenience (opportunity) samples are quick but often biased.
- Capture–recapture: N ≈ number marked × size of second catch ÷ number marked in the second catch.
Worked examples
Example 1
A school has 250 pupils in Year A, 200 in Year B and 150 in Year C. A stratified sample of 60 is taken. How many come from each year?
Answer: 25, 20 and 15