Hello! Last lesson, you distinguished qualitative observations, which describe characteristics, from quantitative observations, which give counts or measurements. Now consider a different question: when an investigation produces a result, how can you tell what was changed and what might have affected that result?
In this lesson, you will identify independent, dependent, and controlled variables in simple investigations. This is an essential skill for planning fair comparisons and explaining them clearly in integrated science. Allow about 35–40 minutes, including time to watch the video and study the reading.
What does each variable do?
Suppose you want to find out whether the amount of water given to seedlings affects their growth. You give different seedlings different amounts of water, then record how tall they grow. Many things about the seedlings and their surroundings could vary, but each factor has a particular role in this investigation.
- The independent variable is the factor you deliberately change: the amount of water.
- The dependent variable is the result you observe or measure: the seedlings’ growth, recorded here as height.
- Controlled variables are relevant factors you keep the same for each seedling, such as the type of soil, the amount of light, and the time allowed for growth.
A variable is a factor that can differ. Calling soil type a controlled variable does not mean soil type can never change; it means you choose to keep it the same in this particular investigation.
The short video develops these roles through a plant-growth example. Its key question is not simply “What factors affect plants?” but “Which one factor are we testing this time?”
Independent, Dependent and Controlled Variables
Watch “Independent, Dependent and Controlled Variables” from Science Ready to see how a broad question about plant growth becomes a comparison in which one factor is deliberately changed.
Watch the three roles for the definitions and the reason for keeping other relevant conditions steady. Then watch the plant example. Focus on why water volume is the independent variable in this test, while soil, sunlight, and temperature are kept the same.
Keeping those other conditions steady makes the comparison fairer. If one seedling receives more water and more sunlight than another, a difference in height cannot clearly be attributed to the water: sunlight is another possible explanation.
The investigation decides each factor’s role
Look at the illustrated comparison below. Both plants are shown growing in pots, but they receive different colours of light.
Here the question concerns light colour, so that is the independent variable. Plant height is the dependent variable because it is the result being compared. To make the comparison meaningful, each plant should receive the same amount of water, the same type and amount of soil, the same growing temperature, and the same daily period of light. The plants should also start at a similar size and be measured after the same length of time. If the lamps provide very different amounts of light, that difference would need attention too.
The illustration shows one plant taller than the other, but a picture alone does not establish that light colour caused the difference. The growing conditions and measurements matter.
Notice how the roles have changed from the video’s plant investigation. When water amount is deliberately changed, it is the independent variable; when light colour is deliberately changed, water amount becomes a controlled variable. No factor is automatically “independent” in every plant experiment. You must read the question being investigated.
Last lesson’s distinction also applies here. “The leaves are pale green” could be a qualitative result; “The plant is 14 cm tall” is a quantitative result. Either could be a dependent variable if it is the outcome the investigator has chosen to observe. Dependent names a factor’s role in the test, while qualitative and quantitative describe how an observation is reported.
Finding the variables in a new investigation
The BBC Bitesize reading gives a second setting: dropping a ball to see how high it bounces. Study its short definitions and then that specific example. As you read, pay attention to the two different heights—one is deliberately chosen, while the other is the result.
Variables - Working scientifically - KS3 Science
Read the relevant parts of “Variables” from BBC Bitesize to reinforce the definitions and see how they apply outside a plant investigation.
Under “Planning an experiment” → “Variables,” begin with the paragraph about factors that can change during experiments. Read the definitions, stopping after the explanation of why other variables are controlled. Then go to “Variables in experiments” → “Experiments 1-4” → “2. Dropping a ball from different heights.” Read the ball example, from its opening sentence through its list of control variables. You can leave the other examples and activities for another time.
In the ball investigation, drop height is independent: the investigator chooses different starting heights. Bounce height is dependent: it is measured after each drop. Although both are heights, they have different jobs. Using the same ball and the same surface helps ensure that a change in bounce height is not simply due to a different ball or floor.
This suggests a reliable way to read an investigation question:
- Ask, “What is deliberately changed or compared?” Name that factor precisely as the independent variable.
- Ask, “What result is observed or measured?” Name that outcome as the dependent variable.
- Ask, “What else could affect that result?” Identify relevant controlled variables and state what must stay the same.
Be specific in an answer. For the ball investigation, “height” is unclear; “height from which the ball is dropped” and “height to which it bounces” show that you know which is which. Likewise, in a plant investigation, “water” may be less precise than “amount of water given to each plant.”
Finally, do not confuse a controlled variable with a control group. A controlled variable is a factor kept the same, such as ball type. A control group, when an investigation uses one, is a group used for comparison. For this lesson, the main task is to identify the three roles of variables, not to assume that every investigation must contain a separate control group.
Key takeaways
To identify variables, begin with the investigation’s question—not with a memorised list of factors. The independent variable is deliberately changed, the dependent variable is the outcome observed or measured, and controlled variables are other relevant factors kept the same for a fairer comparison. The same factor can have a different role when the question changes.
Next, you will learn to select an appropriate measuring instrument and SI unit for a quantity. That will help you record dependent variables, such as plant height or bounce height, accurately.