# how do you wok out stratified sampling with fractions involved

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the sample size for each layer is worked out using this formula:

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'

17 May 2011

sample size for each layer = (size of layer/ size of population) x size of whole sample

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

17 May 2011

So let's say there are 1000 children in the school and 150 children in year 7

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

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!)

17 May 2011

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