Fair testing
Set up simple comparative and fair tests, changing one thing on purpose and keeping other conditions that could affect the measurement the same
What a learner can do afterwards
- Explain what makes a test fair: change one thing on purpose and keep other things that could affect the measurement the same
- Identify the variable to change, the variable to measure, and the variables to keep the same
- Set up and carry out a comparative or fair test with support
1 · Read
A variable is something that can change. In a fair test, choose one thing to change on purpose. Decide what to measure. Keep other things that could affect that measurement the same. This helps you compare the effect of your chosen change.
Mia compares soils for growing beans. She changes the soil type and measures plant height after two weeks. She keeps the bean type, pot size, water amount and light the same, and starts all the seeds at the same time.
Plants can differ even in a fair test. Grow several beans in each soil and repeat the comparison. Record every height. Check whether the plants in one soil are usually taller.
If Mia changes both soil and water, she cannot tell whether a height difference is linked to soil, water or both. To compare soils fairly, she should give each pot the same amount of water.
To compare two heights, subtract the shorter height from the taller height. Mia's plants were 8 cm and 4 cm tall. The difference is 8 minus 4, which is 4 cm.
Choose one change, measure the result, keep other relevant conditions the same, and repeat to check what you find.
Planning a soil test is an original reading written for this dataset.
More to read
Planning a soil test
Which soil helps beans grow taller? Before planting, write down what you will change and what you will measure. Compare soil types and measure plant height after the same number of days.
Use the same kind of bean, the same size of pot and the same amount of water. Give the pots similar light. Start the seeds at the same time. These choices help make the comparison fair.
Plants do not all grow alike. Grow several beans in each soil and record every height, including unexpected results. Repeat the comparison to check whether a similar difference happens again.
Giving one soil more water would add another change. You could not separate the effects of soil and water from that comparison. A fair plan helps you investigate a cause, but one result does not prove it.
2 · Watch
Take it off screen
Where it sits
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.