Name the sampling method
A standard method from Uses and abuses of statistics: the steps and worked examples.
Steps
| Method | How it works | Type |
|---|---|---|
| simple random | every member has an equal chance (draw lots, random numbers) | probability |
| systematic | pick every kth member from a list after a random start | probability |
| stratified | split into groups (strata), sample from each in proportion | probability |
| cluster | split into groups, randomly pick whole groups | probability |
| convenience | whoever is easy to reach | non-probability |
| quota | fill a fixed number per group, not randomly | non-probability |
Worked examples
Example 1: Name the sampling method and check the survey: a school wants students' views on the canteen. It asks the first 30 students in the canteen queue at lunch. Is the sample suitable?
No. It is a convenience sample of students who already use the canteen, so it may be biased towards favourable views and misses students who avoid the canteen. A better plan is stratified random sampling from each form's class lists.
Example 2: Name the sampling method: a school has 400 S1, 300 S2 and 300 S3 students. It picks 50 students in proportion to the size of each form, at random within each form. Name the method and find how many come from each form.
stratified random sampling (the forms are the strata)
total = 400 + 300 + 300 = 1000
S1: 50 × 400/1000 = 20
S2: 50 × 300/1000 = 15
S3: 50 × 300/1000 = 15For the whole topic, read Uses and abuses of statistics: key points, common mistakes and how the DSE tests it.