Planning a fair investigation
Separate the variable you change from the outcome you measure.
Before you begin
- Measurement units
- Calculating an average
Identify variables before collecting data
The independent variable is deliberately changed, and the dependent variable is measured. Control variables are kept as similar as practical so they do not offer alternative explanations. To investigate how light affects a plant's growth, measure a defined growth outcome and keep factors such as species, starting size, water and soil comparable. State how each quantity will be measured.
Use repetition and recognise limits
Repeat measurements or use several comparable samples to see natural variability and reduce the influence of random error on a mean. Repetition does not remove a systematic error, such as a ruler read from the wrong zero. Record unusual observations and investigate them rather than deleting inconvenient results. A conclusion should fit the measured range and the quality of the evidence.
Worked example
Three measurements of a paper helicopter's fall time are 1.8, 2.0 and 2.2 seconds. Find the mean.
Show the worked solution
- Add all recorded times: 1.8 + 2.0 + 2.2 = 6.0 seconds.
- Divide by three repeats to get 2.0 seconds.
- Keep the individual values because they show the variation hidden by the mean.
Mean fall time 2.0 seconds.
Try it yourself
To test how wing length affects fall time, which is the dependent variable?
Common mistakes
- Repeated measurements do not automatically make a biased instrument accurate.
What can you explain now?
Why should all helicopter trials begin from the same height?
Compare with the explanation
Different heights could change fall time independently of wing length.
Controlling release height makes the comparison more informative about wing length. It does not guarantee a perfectly controlled experiment, so use repeats and consistent release technique too.
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Original StudyForA content. AI-assisted checks completed; subject-teacher review is still pending. These lessons are not endorsed by an exam board.
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