8.1 - Variables & Controls
The three kinds of variable
Every experiment has one variable you change, one you measure, and several you keep the same.
The three variables explained
| Variable | What it is | Example: temperature and amylase |
|---|---|---|
| Independent | The one thing you change on purpose | The temperature |
| Dependent | The thing you measure to see the effect | The time for the starch to disappear |
| Control | The things you keep the same | Volume of amylase, volume of starch, pH |
Telling the independent and dependent apart
- Read the aim as "how does X affect Y": X is the independent variable and Y is the dependent variable, every time.
- On a graph, the independent variable goes along the bottom (the x-axis) and the dependent variable up the side (the y-axis).
- The independent variable is the one the student chose to set before starting; the dependent variable is the one they could only find out by measuring.
Controls and valid results
The word "control" is used in two different ways in an experiment - control variables and a control experiment. Don't mix them up.
Control variables keep results valid
- Control variables are the factors you keep the same throughout the experiment - for example, the pH and the volume of amylase.
- Keeping them constant is what makes the results valid: if only the temperature changes and everything else stays the same, any change in the result must be down to the temperature and nothing else.
- Always name a control variable in full: "the volume of amylase", not vague phrases like "the conditions".
A control experiment is run for comparison
- A control experiment is a separate set-up with the factor you are testing left out, run alongside the others for comparison.
- Example: to show that amylase breaks down the starch, set up a tube with boiled amylase that no longer works.
- If the starch breaks down in the ordinary tubes but not in the control, the working amylase must have caused it.
TermDefinition
Independent variableThe variable that you choose to change in an experiment.
Dependent variableThe variable that you measure to see the effect of changing the independent variable.
Control variableA variable that is kept the same throughout an investigation so that only the independent variable can affect the results.
Control experimentA copy of the experiment without the factor being tested, set up alongside it to show that any effect is caused by that factor.
Key points to remember
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