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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

Ask someone else to look at your planned sample before you start your research and ask them if they can see any bias. It can be difficult to look at it objectively yourself, so help and guidance will be useful here.

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
3 population	c to do a particular piece of work, research, etc.
4 census	d not giving a fair representation of the entire population
5 sampling	e typical of people or things in a particular group
6 draw a/the conclusion	f the process of trying to discover all the details or facts about something to discover who or what caused it or how it happened
7 estimate	g to say what you think an amount or value will be, based on the available information to calculate it
8 bias	h to decide what to believe about something after you have considered the facts
9 representative	I the process of selecting part of the whole population for your investigation

Exercise 7

Complete these sentences with words from Exercise 6. You may need to change the form of some words.

1 The sample was not large enough to be ________of the whole population.

2 Poor ________meant that it was difficult to draw conclusions.

3 The study was ________over a three-month period.

4 The study aimed to ________the effects of advertising on children.

5 The sample was ________towards middle aged men.

6 The ________net spend per month was £500.

7 A national ________is carried out in the UK every few years.

8 The ________under investigation was international students in the UK.

9 We can ________the ________that advertising has a significant impact on children’s spending patterns.