Sampling
Population: when you carry out an investigation to find out information about a group of things or people, this group is known as the population.
Some examples of different populations are:
Glossary
population If you refer to a particular type of population in a country or area, you are referring to all the people or animals of that type there.
Census: you would need to carry out a census if you want to find out information about every member of a population. It is easier to carry out a census if you have a small number of people or things in your population, but if the population is very large then it is usually time-consuming, often expensive and sometimes impractical.
Sampling: instead of carrying out a census of the whole population, you could decide to ask questions or observe just a small group of the things or people within that population. This is called sampling. Using a sample can be a lot less expensive, quicker and much easier than carrying out a census. When you have collected information from a sample, it is then possible to draw conclusions and make estimates about the entire population.
Glossary
census A census is an official survey of the population of a country that is carried out in order to find out how many people live there and to obtain details of such things as people’s ages and jobs.
sample A sample of people or things is a number of them chosen out of a larger group and then used in tests or surveys, or used to provide information about the whole group.
Exercise 5
For the investigations listed in the table below, describe the population and decide if it would be practical or impractical to carry out a census to find out information on the whole population.
| Investigation | Sample population | Census: practical or impractical? |
| 1 Finding out how many hours of homework are completed each week by the pupils in a specific class in a school. | ||
| 2 Finding out the number of pets that are owned by adults in London. | ||
| 3 Finding out the number of pages in each book in a large library. | ||
| 4 Finding out how many children in the UK walk to school in the morning. |
Avoiding bias
Using sample data, you can only draw conclusions and make estimates about the entire population if you choose a sample of things or people that represents the whole population fairly. If your sample is not representative of the population, then it is known as a biased sample.
The table below describes some samples taken for different investigations, and explains why these samples are not representative of the population. Can you work out why the sample is biased for the last two investigations?
Glossary
biased If someone is biased, they prefer one group of people to another, and behave unfairly as a result. You can also say that a process or system is biased.
| Investigation | Sample | Why the sample is biased |
| The value of all houses in Bristol | Houses in one street in Bristol | Houses in the same street are likely to be similar in size, so the sample only fairly represents that street rather than all the houses in Bristol, which will vary considerably in size and therefore in value. |
| The favourite football team of men aged 20–40 in the UK | Men aged 20–40 who go to a football match between Manchester City and Manchester United in February | |
| People’s opinion about whether driving a car is the best way to travel to work | Drivers who have just parked their car in a company car park |
Tip
Exercise 6
Match the words or phrases on the left to a definition on the right.
| 1 carry out | a the group you want to investigate |
| 2 investigation | b a study of the whole group |