Every fair test sorts its factors into three groups: one to change, one to measure, and the rest held constant. A control set-up then gives a comparison.
This lesson belongs to biological investigation and follows writing a testable hypothesis.
How do I sort the variables?
Ask three questions about any experiment. What does the student change? What does the student measure? What must stay the same?
| Type | Question | How to keep it controlled |
|---|---|---|
| Manipulated | What do I change on purpose? | Set different values, such as 10, 20, 30 °C |
| Responding | What do I measure? | Record with a consistent instrument and unit |
| Fixed | What must not change? | Use the same volume, time, mass or species each time |
Worked example: a germination experiment
Situation. A student places 10 mung bean seeds on damp cotton wool in each of four dishes. The dishes are kept at 15 °C, 25 °C, 35 °C and 45 °C. After 3 days the student counts the seeds that have germinated.
Step 1. The temperature is changed on purpose, so the manipulated variable is temperature.
Step 2. The number of seeds that germinated is measured, so the responding variable is the number of seeds germinated.
Step 3. The fixed variables are the number and type of seeds, the amount of water, and the duration of 3 days. Light exposure is another fixed variable.
Step 4. Every dish is treated the same except for temperature, so the dishes can be compared with one another. A control for water would be one extra dish kept dry at 25 °C, to show that germination needs water.
The mistake that costs marks
The slip is to write “control” when you mean “fixed variable”. “Control the volume of water” is fine as an instruction, but the answer should list the variables kept constant and explain them.
Write: “Fixed variable: volume of water, 20 cm³ in each dish.” This names the variable and the value, so the marker can see it is constant.
Check yourself
A student tests whether different light colours affect the height of radish seedlings after 7 days. Identify the manipulated variable, the responding variable and two fixed variables.
Answer
Manipulated variable: the colour of light. Responding variable: the height of the seedlings after 7 days, in mm.
Fixed variables: the type and number of seeds, the volume of water, the temperature and the duration. Any two earn the marks.
What to study next
Next, plan how to record results in planning tables and biological drawings. Then try the biological investigation practice. For a teacher to give you fresh set-ups, see online one-to-one Biology tuition.