Controlling Variables and Experiments
Understand Controlling Variables and Experiments for GCSE Statistics with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA and Edexcel. Experiments use control groups, matched pairs and controlled variables to make fair comparisons.
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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
An experiment investigates whether changing one variable causes a change in another. Controlling variables is what makes the conclusion valid.
The explanatory variable is the one deliberately changed. The response variable is the one measured. All other variables that could affect the response must be kept constant — these are the control variables.
If a variable is not controlled, it becomes an extraneous variable and the results cannot be trusted, because any change might have been caused by it rather than by the explanatory variable. A control group receives no treatment and provides a baseline against which the treated group is compared.
Revision notes
The three types of variable
Explanatory variable: the one deliberately changed, sometimes called the independent variable.
Response variable: the one measured, sometimes called the dependent variable. Control variables: all others kept constant so they cannot affect the result.
Extraneous variables
An extraneous variable is one that is not controlled but could affect the response variable.
If one is present, any change observed might have been caused by it rather than by the explanatory variable, so no valid conclusion can be drawn.
Control groups
A control group receives no treatment, providing a baseline for comparison.
Without it there is nothing to compare the treated group against, so it is impossible to tell whether the treatment had any effect at all.
Key points
- The explanatory variable is deliberately changed.
- The response variable is measured.
- Control variables are kept constant.
- An uncontrolled variable is extraneous.
- Extraneous variables invalidate conclusions.
- A control group provides a baseline.
Worked examples
Example 1
A student investigates whether fertiliser affects plant growth. Name the explanatory variable. [1 mark]
Working
Example 2
Explain why other variables must be kept constant in an experiment. [2 marks]
Working
Example 3
Explain the purpose of a control group. [2 marks]
Working
Common mistakes
Confusing explanatory and response variables.
Explanatory is changed; response is measured.
Forgetting to name control variables.
Questions often ask for specific variables to keep constant.
Saying a control group receives a different treatment.
It receives no treatment, providing a baseline.
Concluding cause without controlling variables.
An uncontrolled variable makes any causal conclusion invalid.
Exam tips
- Identify which variable is changed and which is measured.
- Name specific control variables rather than saying 'everything else'.
- Explain the control group as a baseline.
- Link uncontrolled variables directly to invalid conclusions.
Key terms
- Explanatory variable
- The variable deliberately changed.
- Response variable
- The variable measured.
- Control variable
- A variable kept constant.
- Extraneous variable
- An uncontrolled variable that could affect the result.
Related topics
Written and reviewed against the current AQA and Edexcel specifications. Spotted an error? Let us know.