# how do you wok out stratified sampling with fractions involved

how do you wok out stratified sampling with fractions involved

the sample size for each layer is worked out using this formula:
chloe
17 May 2011
a stratisfied sample is sampling different 'layers' for example, take samples from different age group with each age group being a 'layer'
chloe
17 May 2011
sample size for each layer = (size of layer/ size of population) x size of whole sample
chloe
17 May 2011
For example you might be asked to take a stratified sample of 50 children in a whole school so in order to make sure that you get a proportionate number of students in each year in the sample you would have to use the formula
chloe
17 May 2011
So let's say there are 1000 children in the school and 150 children in year 7
chloe
17 May 2011
150 is the size of the layer, 1000 is the size of the whole population and 50 is the size of the sample
chloe
17 May 2011
So using the formula above we would take (150/1000) x 50 = 7.5. You would probably round this up to 8 students for this year group (normally questions don't end up with answers having decimal places, I just didn't think the question through when I started!)
chloe
17 May 2011