Variables, controls and fair tests
A valid investigation changes one relevant quantity while controlling other factors that could affect the result.
Change one independent variable, measure the dependent variable and hold relevant controls constant.
By the end of this lesson, you can:
- identify independent, dependent and control variables;
- choose a useful range and interval;
- describe how control variables are held constant;
- recognise confounding variables.
Three variable roles
The independent variable is deliberately changed. The dependent variable is measured in response. Control variables are kept constant because they could otherwise alter the dependent variable.
Range and distribution
Use the widest safe and practical range, with values distributed across it. Very narrow ranges make trends and gradients less reliable.
Operational controls
Do not merely say 'keep temperature constant'. Explain how: allow apparatus to cool between readings, monitor with a thermometer, or use a water bath.
WORKED EXAMPLEReveal method
When testing resistance against wire length, keep material, diameter and temperature constant.
Try another worked problem
Attempt the question yourself, then reveal the examiner-style solution when you are ready.
Control variable
When testing pendulum period against length, name two controls.
CHECK YOUR ANSWERReveal solution
Keep bob mass/shape and initial small angle constant; use the same release method.
Mastery check
Which quantity is deliberately changed by the experimenter?