Experimental Design
Experimental design is the process of planning a scientific or statistical experiment so that it produces fair, reliable data and isolates the effect of the variable being tested. In GCSE Statistics this covers control groups, randomisation and reducing bias, so that any difference in results can be confidently linked to the variable under investigation rather than to chance or an outside factor.
Before you start
Make sure you're comfortable with these topics first:
Method
- Identify the independent variable (the one you change) and the dependent variable (the one you measure).
- Identify control variables that must be kept the same for every trial, so they cannot unfairly affect the result.
- Use a control group that does not receive the treatment, so it can be compared with the group that does.
- Assign participants or items to groups at random, to avoid bias in who ends up in each group.
- Use as large a sample as is practical, and repeat trials to check the results are reliable.
- Consider using a blind or double-blind procedure where relevant, so participants or researchers cannot influence the result.
Worked example
A company wants to test whether a new fertiliser increases tomato plant height. Describe a fair experimental design, including a control group.
- Identify the variables: independent variable is whether fertiliser is used, dependent variable is plant height after a fixed time.
- Identify control variables to keep constant: same plant species, same amount of water, same amount of sunlight, same size of pot.
- Take a large sample of plants and randomly assign half to the fertiliser group and half to a control group with no fertiliser.
- Grow all plants under the same conditions for the same length of time, changing only the fertiliser.
- Measure the height of every plant at the end and compare the mean height of the two groups.
Practice questions
Try each question, then tap to reveal the answer.
Q1What is meant by the independent variable in an experiment?Show answer
Answer: The variable that the experimenter deliberately changes or controls.
Q2What is a control group used for?Show answer
Answer: To give a baseline for comparison, showing what happens without the treatment being tested.
Q3Give one reason why participants should be assigned to groups at random.Show answer
Answer: To avoid bias, so pre-existing differences between participants are spread evenly between the groups.
Q4A scientist tests a new plant food but only uses it on the healthiest-looking plants. Explain why this is a flawed design.Show answer
Answer: The sample is not randomly assigned, so results cannot be fairly compared with a typical control group.
Q5What is meant by a control variable?Show answer
Answer: A variable that is kept the same for every group or trial, so it cannot affect the result.
Q6In a medicine trial, half the patients receive the real drug and half receive a dummy pill, and neither the patients nor the doctors know who received which until after results are recorded. What is this type of trial called, and why is it used?Show answer
Answer: A double-blind trial; it prevents the patients' or doctors' expectations from biasing the results.
Exam-style questions
Written in the style of a GCSE Statistics exam paper, with a full mark scheme.
Explain why a sample size of 4 plants is not sufficient to draw a reliable conclusion in an experiment.
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A gardener wants to test whether talking to plants increases their growth. Describe a fair test, including how a control group should be used.
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A researcher is investigating whether a new teaching method improves test scores. She lets students choose which method they use. (a) Explain one problem with this design. (b) Describe two improvements that would make the experiment fairer.
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See real GCSE Statistics past-paper questions, with official mark schemes →
Free printable worksheet
Want more practice on paper? Download the experimental design worksheet pack - 15 pages of exam-style questions with a full mark scheme. One email opens every download in this browser for 14 days - no account, no card. Print it for personal and classroom use.
This topic is chapter 10 of GCSE Statistics Foundation Workbook 1 and chapter 10 of GCSE Statistics Higher Workbook 1, the whole course as one free printable PDF.
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