Practical Example
In general the size of the sample in each stratum is taken in proportion to the size of the stratum. This is called proportional allocation. Suppose that in a company there are the following staff:
- male, full-time: 90
- male, part-time: 18
- female, full-time: 9
- female, part-time: 63
- Total: 180
and we are asked to take a sample of 40 staff, stratified according to the above categories.
The first step is to find the total number of staff (180) and calculate the percentage in each group.
- % male, full-time = 90 / 180 = 50%
- % male, part-time = 18 / 180 = 10%
- % female, full-time = 9 / 180 = 5%
- % female, part-time = 63 / 180 = 35%
This tells us that of our sample of 40,
- 50% should be male, full-time.
- 10% should be male, part-time.
- 5% should be female, full-time.
- 35% should be female, part-time.
- 50% of 40 is 20.
- 10% of 40 is 4.
- 5% of 40 is 2.
- 35% of 40 is 14.
Another easy way without having to calculate the percentage is to multiply each group size by the sample size and divide by the total population size (size of entire staff):
- male, full-time = 90 x (40 / 180) = 20
- male, part-time = 18 x (40 / 180) = 4
- female, full-time = 9 x (40 / 180) = 2
- female, part-time = 63 x (40 / 180) = 14
Read more about this topic: Stratified Sampling
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