Year 6 Science: Planning a fair test
Part of the Year 6 Science working scientifically (years 5-6) chapter. Chapter overview · whole chapter on one page
Planning a fair test
In a fair test, we plan the test around variables. A variable is anything that could change in the test. There are three kinds:
- the changed variable is the one thing we change on purpose;
- the measured variable is the thing we measure to get our results;
- the fixed variables are all the other things, which we keep the same.
A fair test changes only one variable. If we change more than one thing, we cannot tell which one caused the result.
Key ideas
- A fair test changes only one variable.
- The changed variable is the one we change on purpose.
- The measured variable is what we measure to get the results.
- The fixed variables are kept the same, so the test is fair.
Method. Find the one thing that is changed on purpose.
- List the three choices. The type of towel, the amount of water and the time are the three choices.three choices
- Pick the one we change on purpose. Aisha and Tom choose different towels to compare, so the type of towel is changed.type of towel
- Write the answer. the type of towel
Check. The water and the time stay the same for every towel. ✓
Watch out. The amount of water and the time are fixed variables, not the changed variable.
Method. Check how many variables change.
- Compare the two tests. Two things change: the type of towel and the amount of water.towel and water
- Decide. A fair test changes only one variable, so this test is not fair.not fair
- Write the answer. no
Check. Only the amount of water should change in a test of the water. ✓
Method. Remember the rule for a fair test.
- Read the rule. A fair test changes only one variable.one variable
- Count the changed variables. Every other variable is a fixed variable, kept the same.one changed, the rest fixed
- Write the answer. 1
Check. Every other variable is a fixed variable, kept the same. ✓
Watch out
Watch out: the variable you measure is not the same as the variable you change. In the paper towel test you change the towel, and you measure the water absorbed.
Exam tip
Exam tip. When a question asks for a fair test, name the changed variable first, then at least one fixed variable.
Try it (with your teacher, or a grown-up at home). Put three small squares of different paper towel, or three pieces of kitchen paper of the same size, into three dishes with the same amount of water. Keep the size and the water the same. Predict which towel will soak up the most water, then leave them for one minute and look. Write down which variable you changed and which you kept the same.
Exercise 1A Planning a fair test
Method. Read the definition of the changed variable.
- Read the definition. The changed variable is the one thing we change on purpose.changed variable
- Write the answer. changed variable
Check. The changed variable is set by the person doing the test. ✓
Method. Read the definition of the measured variable.
- Read the definition. The measured variable is what we measure to get the results.measured variable
- Write the answer. measured variable
Check. For the paper towel test, the measured variable is the water absorbed. ✓
Method. Read the definition of the things kept the same.
- Read the definition. The fixed variables are kept the same in every test.fixed variables
- Write the answer. fixed variables
Check. Fixed variables make the test fair. ✓
Method. Think about what must be the same for every towel.
- Think about the test. The amount of water, the size of the towel and the time must be the same for every towel.the same for every towel
- Write one fixed variable. For example, use 50 mL of water for every towel.50 mL each time
- Write the answer. Accept any one of: the amount of water, the size of the towel, the time the towel is left in the water.
Check. Any of these would be a fixed variable. ✓
Method. Read the rule for a fair test.
- Read the rule. A fair test changes only one variable.only one
- Write the answer. one
Check. Changing two variables makes the result hard to explain. ✓
Method. Count the variables that change.
- Read the sentence. Ruby changes two things: the ramp and the car.two things
- Decide. Changing two things is not a fair test, so the answer is no.no
- Write the answer. no
Check. Only one thing should change, for example only the ramp. ✓
Method. Decide which thing Sam changes on purpose.
- Find the change. Sam chooses the height he drops the ball from.height of drop
- Find the measurement. The height of the bounce is what he measures.measured
- Write the answer. height of drop
Check. The height of the bounce is the measured variable. ✓
Method. Think about what else could affect the plant.
- Think about the plant. The type of plant, the water and the pot could all change growth.type, water, pot
- Write one fixed variable. Using the same type of plant keeps the test fair.same plant
- Write the answer. Accept any one of: the type of plant, the amount of water, the size of the pot.
Check. Keeping them the same means the sunlight is the only changed variable. ✓
Method. Find the thing Aisha changes on purpose.
- Read the plan. Aisha changes the temperature of the water on purpose.temperature
- Write the variable. So the changed variable is the temperature of the water.temperature of the water
- Write the answer. temperature of the water
Check. The time for the sugar to dissolve is the measured variable. ✓
Method. Count the changes in her plan.
- Count. Priya plans to change three variables.three
- Explain the problem. If the result changes, we do not know which change caused it.which change
- Write the answer. Accept any answer that says she changes three things at once, so she cannot tell which one made the difference.
Check. A fair test changes only one variable. ✓
Method. Think about cause and effect.
- Think about one changed variable. If only one thing changes, then any change in the result must come from it.one cause
- Compare with three. With three changes, we cannot tell which change caused the result.three causes
- Write the answer. Accept any sentence that says that with only one changed variable, any change in the results must come from that variable.
Check. One changed variable means one possible cause. ✓
Method. Check that only one variable changes.
- Read the test. Only the amount of water changes; the towel and the time stay the same.only water
- Decide. Because only one variable changes, the test is fair, so the answer is yes.yes
- Write the answer. yes
Check. The same towel and the same time were used. ✓
Method. Plan the test with one change.
- Choose the changed variable. We change the surface, on purpose.surface
- Choose fixed variables. Keep the ramp, the car and the starting height the same.ramp, car, height
- Write the answer. Accept any answer that names the surface as the changed variable, and one fixed variable such as the ramp, the car or the starting height.
Check. Only the surface changes, so the test is fair. ✓