Let’s say I’m being paid by a company to find out what type of chocolate everyone in the UK prefers. Well, I can’t ask everyone, so I’ll ask a much smaller group of people. That group is called a sample of the population.

The second thing I need is a ‘sampling strategy’, i.e. how am I going to choose the people to ask?

There are four strategies that you need to know:

  • Random Sampling

Individuals are selected at random and have an equal chance of being selected.

A table of random numbers needs to be used in order to carry this type of sampling, or a calculator may be used.

Example:

10 pupils need to be selected from a year group of 250 students.

First we need to number the pupils, since there are 250 students we start from 000 (if there were only 50 students, we would start our numbering from 00). So the pupils would be numbered 000, 001, 002, 003, 004, 005, ....., 249. 

After numbering the pupils we can use a calculator to randomly generate 3-digit numbers. For example: 112 143 172 222 206 094 046 148 010 197 058 149 101 151 247 176 225

We then select the student with the corresponding random number generated to select our random sample.

  • Quota Sampling

Quota sampling involves splitting the population into groups and sampling a given number of people from each group.

For instance, an interviewer needs 50 men and 50 women aged between 25 and 40 to take part in his survey about which fast food restaurant they like best. The interviewer doesn't need to worry about randomly selecting his participators, he just needs to fill his quota. i.e. as long as he gets 50 men and 50 women aged 25 and 40 he is fine.

This method often leads to bias. If the interviewer decided to ask people who have just walked out of a fast food restaurant, they are more likely to name that restaurant as their favourite place. 

  • Systematic Sampling

Instead of using random numbers to select individuals, a random starting point is selected then every nth individual is selected.

For example, say we have 10 pupils to sample out of 100. Each pupil, like in random sampling is numbered, from 000 to 099. Now we select a random starting point (e.g. 034) and then select every 10 students from that starting point you come across. So we would chose pupils numbered 034, 044 ,054 ,064 ,074 ,084 ,094 ,004, 014 and 024 to form our sample.

  • Stratified Sampling

If the population falls into several different groups, choose some of the groups to sample from, then pick a rnadom selection from those groups. See the 'Use Stratified Sampling' page http://cribbd.com/learn/maths/data-handling/use-stratified-sampling for more details

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