Idea 12of 22Builds onNames in a hat
Split into groups first
How can you make sure every group gets a say?
Today you'll discover
- how to split a population into groups
- how much to draw from each
- why the mix comes out right
Think firstTheo wants bus riders and walkers heard
Theo wants bus riders and walkers both heard. What should he do first?
Decide first, then press Next. Watch the grid pull apart into two groups.
The same 60 students. The 20 squares, bus riders, stay on the left as the 40 circles, walkers, slide right; dashed outlines go round each group. Then 4 bus riders and 8 walkers are ringed. At the right, bars compare bus riders: 4 of 12 in the sample, 20 of 60 in the school.
See the whole ideathe big idea and the key word, in one place
The big idea
Stratified random sampling splits the population into groups first, then draws at random from each.
Split, then draw
Split, then draw the same share, 1 in 5, from every group.
- Population60
- Splitbus 20, walk 40
- 1 in 5 of each4 + 8
- Sample12
Easy to get wrongInside each group the draw must still be random. Splitting and then asking the nearest students shuts people out again.
Key word
- stratified random sampling
- splitting the population into groups first, then choosing at random inside each group
Ready to use it?
- Name stratified random sampling from a description.
- Work out how many to draw from each group.