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Learning outcomes
- Identify independent, dependent and controlled variables.
- Explain why a control variable matters.
- Recognise when a test is not valid.
- Choose a suitable range and spacing of values.
- Write variables in measurable form.
6.1 Independent and dependent variables
The independent variable is deliberately changed. The dependent variable is measured as the response. Both should be expressed as measurable quantities, not vague labels.
In a wire experiment, ‘length of wire’ is a suitable independent variable and ‘potential difference’ or ‘resistance’ may be dependent variables. ‘Electricity’ is not a measurable variable.
6.2 Controlled variables
Controlled variables are factors that could affect the dependent variable and therefore must be kept constant to isolate the effect of the independent variable. A valid experiment tests what it claims to test.
When investigating wire length and resistance, material and diameter must remain constant. Temperature should also remain approximately constant because resistance changes when the wire heats.
6.3 Explaining how to control
Naming a control variable is only the first step. State how it is controlled: use the same wire throughout; use equal volumes measured with the same cylinder; start each cooling test at the same temperature; keep the lens and screen vertical.
Some variables cannot be held perfectly constant but can be monitored or their effect reduced. Room temperature can be measured, experiments can be performed close together in time, and draughts can be avoided.
6.4 Range and spacing
Choose enough values to show a pattern and support a graph, usually at least five or six. Spread values across the largest safe and useful range. Even spacing is often convenient, but the physical response and apparatus limitations may justify unequal spacing.
A very narrow range produces a small change in the dependent variable and makes the trend difficult to distinguish from measurement scatter. A range that overloads apparatus or changes the physical regime is also unsuitable.
6.5 Fair test versus comparison
A fair test does not mean every quantity is equal. The independent variable must change and the dependent variable may change. Other relevant variables are controlled.
When comparing two materials, material is the independent variable. Dimensions, starting temperature, applied load or other relevant factors must be made equal so the comparison is meaningful.
Worked examples
Cooling investigation
Independent: type or thickness of insulation. Dependent: temperature decrease in a fixed time. Controls: water volume, initial temperature, container, thermometer depth, lid and room conditions.
Pendulum investigation
Independent: pendulum length. Dependent: period. Controls: bob, release angle, timing method and pivot arrangement.
Practical focus
Investigation or training activity
For five syllabus experiments, create a three-column variable table. Add a fourth column explaining exactly how each controlled variable is kept constant.
Examination guidance
- Write variables as quantities that can be measured.
- Do not call time a control if time is the quantity being changed or measured.
- Explain how each important control is achieved.
- Choose a range wide enough to show a measurable change.
- Avoid changing two independent variables at once.
Check your understanding
- What is the dependent variable?
- Why must wire diameter be controlled in a length–resistance test?
- How many values are normally useful for a graph?
- Is a fair test one where all variables stay constant?
Answers
- The measured response.
- Diameter also affects resistance and would confound the effect of length.
- Usually at least five or six.
- No; the independent variable changes and the dependent variable may respond.
