Stratified Sampling - Practical Example

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