Random sampling

It is easy to misuse the word 'random'. You might say that you carried out your survey by questioning people 'at random', but what does it really mean to choose someone at random?

Random sampling means that members of a 'population' have equal chances of being selected.

To carry out this type of sampling, you will need to use a table of random numbers. Random numbers can also be generated using a calculator or computer. These can then be listed.

For example, if these random numbers are generated by a calculator:

017, 029, 300, 914, 037, 849, 111, 559, 333, 400, 598, 255.

Use these to make up the list of numbers to select a sample:

0 1 7 0 2 9 3 0 0 9 1 4 0 3 7 8 4 9 1 1 1 5 5 9 3 3 3 4 0 0 5 9 8 2 5 5.

Example

Of 1000 pupils in a school, 50 are to be questioned about their favourite pop group. How should the pupils be chosen?

Solution

The pupils should be numbered 000, 001, 002, 003, ....., 999.

You can use a calculator to generate random numbers. Three-digit random numbers can then be used to choose 50 pupils.

Remember: when carrying out a random sample, you must ensure that all possible samples are equally likely to be chosen.