Statistics - sampling methods


certain cereal manufacturer wants to carry out a survey in the county Middlesex to determine the most popular breakfast cereal among school kids. Asking each and every child about their choice is next to impossible; the number of kids runs into millions.

It is what we call in statistics, a vast population. When we are faced with a situation of this kind, it makes sense to concentrate on a manageable group of kids to get a true picture of their needs.

This small group is called a sample and the process is known as sampling.

 We must be very careful about this small group of individuals – sample.
At the very beginning it must fulfil the following requirements: 

With setting our sight on these points, the next task is to number the whole population in a certain way; the letters of the first names and surnames can be considered in this numbering process. Each and every member of the population now has a number. 

The size of the sample is normally between 5%-10% of the population.

Taking a Sample from a Population - interactive



There are 600 students in Heathfield Grammar School. We want to carry out a survey to find out how they travel to school every morning. - the sample size = 10 % of 600 = 60 

The population – in this case the kids in Heathfield Grammar School – are numbered in the following way: 

  1. Adam Ashford
  2. Adrian Burton
  3. Ajit Bra
  4. Balvinder Chopra
  5. Azis
  6. Hamza
  7. ...
  8. ...
  9. Melani Yanks
  10. Nigel Zamir
  11. Owen Bright
  12. ...
  13. ...
  14. Raymond Hill
  15. Tim Sinclair

Now we have to choose 60 members from the above list. It is the process of sampling. This can be done in three different ways: 

Random Sampling

Generate 60 random numbers by any of the following methods:

Regular Pentagon  

Throw the three spinners and take the number on display in order – 599, 400, 398 etc. Spin all three 60 times. You can get sixty random numbers by any one of these three methods. It is random sampling.

Stratified Sampling

In this method, we divide the population into smaller units: we can divide them on the basis of ethnicity of the members – just one way of grouping the population. Then the members for the sample are chosen from these individuals, of course in proportion to the size of each unit.


The population of Heathfield Grammar School is grouped in the following way:

White Caucasians150
Sikhs 180
Muslims 120

The members for the sample in proportion to the size of the groups are chosen as follows:

White Caucasians10% of 150 = 15
Sikhs 10% of 180 = 18
Muslims 10% of 120 = 12
Afro-Caribbean10% of 90 = 9
Other10% of 60 = 6

How do you choose these members from each group? You can use random sampling for that purpose.

Selective Sampling

In this method, a random number is generated by some means – calculator, spinner or a simple computer programme.

If the number turns out to be 5, then every 5th member must be picked up for the sample.



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