Stratified Sampling

Stratified Sampling

1. A stratified sample is made up of different 'layers' of the population

   • for example, selecting samples from different age groups.

2. The sample size for each layer is proportional to the size of the 'layer'.

   • Sample size for each layer = size of whole sample/size of population × size of layer
     • So we look at what fraction of the whole population do we want. Then take that fraction of each layer.

3. example

   • 50 pupils in a school of a 1000 pupils were asked what music they like
     • make sure the survey is accurate you will need a range of pupils across the year groups - different layers.
       • year 7's may like different music to year 11's
         • To work out sample size for year 7:
           • There are 180 students in year 7 - this is the size of the layer.
             • There are 1000 pupils in the school - this the size of the whole population.
               • You want answers from 50 people in total - this is the size of the whole sample.
           • 50/1000 × 180 = 9