Controlling variables (age 11+)
Form a testable scientific hypothesis linking an independent variable to a predicted outcome, plan a full investigation identifying independent, dependent, and control variables, sample size, and risk assessment
What a learner can do afterwards
- Writes a testable prediction linking an independent variable to an expected dependent-variable outcome, supported by scientific reasoning
- Identifies and labels the independent variable, dependent variable, and at least three control variables
- Plans repeat readings and an appropriate sample size, and identifies relevant hazards with control measures
1 · Read
You have planned fair tests before. A hypothesis is a possible explanation that can be tested. It can lead to a prediction. In this lesson, name the condition you will change, the outcome you expect to measure and a scientific reason for that expectation. You can join these with 'I predict ... because ...', but scientists do not have to use those exact words.
Priya predicts that sugar will dissolve faster in warmer water. Water particles move faster when warmed, which helps sugar mix into the water. She will change water temperature and measure dissolving time in seconds. She will use the same sugar mass, water volume and stirring method for each trial.
The independent variable is what you change on purpose: water temperature. The dependent variable is the outcome you measure: dissolving time. Control variables are relevant conditions kept the same. Priya keeps sugar mass, water volume and stirring method fixed. Changing both sugar mass and temperature would make it hard to separate their effects on dissolving time.
Sample size counts separate objects, not readings. Several plants at each light level let you compare variation between plants. Repeated height readings of each plant check measurement variation. Explain your planned counts using likely variation and available time. For the sugar test, three fresh trials at each temperature are a starting plan, not a guarantee. Keep unusual results and investigate them. More objects or repeats cannot repair a faulty method.
A risk assessment names a hazard, the harm it could cause and a way to reduce the risk. Hot water can scald skin. An adult should handle and pour hot water for this activity. Use suitable containers on a stable, heatproof surface. Never use a finger to test the temperature. Follow your teacher's safety instructions.
Plan the sample and repeats together. Multiply plants by readings per plant to check the total. With a limit of 20 readings and four readings per plant, five plants fit: 5 × 4 = 20. Divide the reading limit by readings per plant to find the largest sample. Count whole plants only; do not exceed the limit.
Each row represents one plant. Each dot represents one planned height reading.
A strong investigation starts with a hypothesis that has a reason, then a fair test that changes only one thing, repeats its readings, and plans for safety.
This lesson, its questions and its flashcards were written for this dataset.
These readings and flashcards were written for this dataset.
More to read
Planning what a test could show
A hypothesis gives a possible explanation that can be tested. Before collecting results, use the explanation to predict what you expect to observe.
Name the condition you will change and the outcome you will measure. Explain why you expect that outcome. The reason should concern the science, rather than the result you hope to get.
Also consider a result that would challenge your prediction. If the outcome stays the same, record that too. A useful investigation does not need to produce the predicted result.
Afterwards, compare the evidence with the prediction. Check the method before making a claim about what caused the result. Supporting evidence can strengthen an explanation without proving that it is the only possible explanation.
Writing down the variables
A plan should let another person identify what changes and what stays fixed. In a sugar-dissolving test, write down the water temperatures you will compare. These are values of the independent variable.
Record the time the sugar takes to dissolve. This measured outcome is the dependent variable. It might change with temperature, or the test might show no clear difference.
Write the control conditions beside the method: equal sugar masses, equal water volumes and the same stirring method. Explain how you will keep each one fixed.
Changing the sugar mass and temperature together would give two possible explanations for a time difference. Keeping relevant conditions fixed helps separate their effects, although chance variation and measurement problems can still affect results.
Objects, readings and risks
Suppose a class studies six plants. That is a sample of six plants. Measuring each plant twice produces twelve readings of those same six plants.
Separate plants can differ from one another. Repeat readings can also vary. Your plan should say how many objects you will study and how many readings or trials you will take under each condition.
Choose numbers that fit the question, available time and expected variation. There is no repeat count that makes every method trustworthy. A larger sample also cannot repair faulty equipment or an unfair comparison.
List risks before beginning. Name the hazard, the possible harm and the action that reduces the risk. In a hot-water activity, follow teacher instructions and have an adult handle and pour the hot water.
2 · Watch
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Where it sits
Learn first
Optional connections
Where this leads
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.