Sampling Bias
Master Sampling Bias for GCSE Statistics with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA and Edexcel. Bias occurs when a sample is not representative of the population, distorting the conclusions.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA and Edexcel specifications. Every worksheet comes with a full mark scheme.
Topic overview
Bias is a systematic tendency for a sample to misrepresent the population. It is not the same as random variation — bias pushes results consistently in one direction.
Sampling bias arises when some members of the population are more or less likely to be selected than others. Surveying only at a train station excludes people who drive. Surveying only during the day excludes those at work.
Non-response bias arises when those who do not respond differ systematically from those who do. If a survey about satisfaction is only returned by dissatisfied customers, the results overstate dissatisfaction. Bias cannot be fixed by increasing the sample size, because a larger biased sample is still biased — this point is examined regularly.
Revision notes
What bias is
A systematic tendency for a sample to misrepresent the population.
It differs from random variation because it pushes results consistently in one direction rather than scattering them either side of the true value.
Sources of bias
Sampling bias: some members are more or less likely to be selected. Surveying at a station excludes drivers.
Non-response bias: those who do not respond differ from those who do. Question bias: a leading question pushes respondents towards an answer. Interviewer bias: the interviewer chooses who to approach.
Why a bigger sample does not help
Increasing the sample size reduces random variation but does not remove bias.
A larger biased sample is still biased in the same direction — it simply gives a more precise estimate of the wrong value. The method must be changed, not just the size.
Key points
- Bias systematically misrepresents the population.
- It pushes results consistently in one direction.
- Sampling bias excludes part of the population.
- Non-response bias arises when non-responders differ.
- Leading questions cause bias.
- A larger sample does not remove bias.
Worked examples
Example 1
A survey is carried out at a train station during the morning rush hour. Give one reason why the sample may be biased. [1 mark]
Working
Example 2
Explain why increasing the sample size does not remove bias. [2 marks]
Working
Example 3
Explain what is meant by non-response bias. [2 marks]
Working
Common mistakes
Saying a larger sample fixes bias.
It reduces random variation only. The method must change to remove bias.
Confusing bias with a small sample.
A small sample is imprecise; a biased sample is systematically wrong.
Giving a vague answer such as 'it is unfair'.
Say specifically who is excluded or over-represented.
Forgetting non-response as a source.
It is a standard source and frequently examined.
Exam tips
- Name specifically who is excluded or over-represented.
- Remember bias is systematic, not random.
- State that a bigger sample does not remove bias.
- Learn several distinct sources of bias.
Key terms
- Bias
- A systematic tendency to misrepresent the population.
- Sampling bias
- Bias from some members being less likely to be selected.
- Non-response bias
- Bias from non-responders differing from responders.
- Systematic
- Consistently in one direction, rather than random.
Related topics
Written and reviewed against the current AQA and Edexcel specifications. Spotted an error? Let us know.