How to identify variables in a school experiment

Write the question as “How does changing [one factor] affect [a measured outcome]?” The factor you deliberately change is the independent variable. The outcome you measure is the dependent variable. Replace broad words such as “size” or “performance” with the particular dimension or result you intend to use.

For each variable, specify how it will be set or measured and include a unit where appropriate. “Ramp height in centimetres” is more precise than “ramp”. “Distance travelled on the floor in centimetres” is more precise than “how well the car moves”. If the question comes from an assignment, keep its intended outcome rather than substituting something easier to measure.

Control variables are other relevant factors you aim to keep consistent across the conditions. List factors that could affect the outcome, then write how you will manage each one. “Same car” is a practical instruction; “everything stays the same” leaves too much unstated.

Check whether the method changes more than one factor at once. If you compare a low ramp with one car on a smooth floor against a high ramp with another car on carpet, the travel distance could reflect several differences. That setup makes it difficult to attribute the comparison to ramp height. Change the method or narrow the conclusion to match what was actually compared.

Imagine a practice investigation asking how the height of the raised end of a toy-car ramp affects the distance the car travels on the floor after leaving it. Proposed heights are 5 cm, 10 cm and 15 cm. These are invented planning choices, not official instructions or results from an experiment.

Independent variable: height of the raised end, measured vertically from the floor. Dependent variable: distance along the floor from a marked line at the ramp exit to the car’s front when it stops. Controls: use the same car, ramp length and floor surface; release from the same marked position on the ramp without a push; use the same distance reference points each time.

Raising one end of a ramp of fixed length also changes its angle. That is part of this chosen way of changing height, so do not claim the ramp angle stays constant. A plan can contain related quantities that cannot all be held fixed. Check the geometry of the setup before promising controls the method cannot maintain.

The plan contains no prediction of the travel distances. Those values belong in a results table only after measurement. For a real practical, use the equipment, conditions and procedure agreed with your teacher rather than treating this example as an assigned method.

State the start and end points of the measurement, the instrument and the rule for reading it. For the ramp example, using the front of the car in one trial and its rear in another would change the reported distance. A consistent reference makes the readings comparable.

Plan repeated trials for each condition if the task permits them, resetting the setup each time. Repeats can show variation in the readings; they cannot repair a control that was changed between conditions. Record individual readings and any departures from the method so a later comparison can account for them.

Use four lines: “Change: [factor, settings and how to set them]. Measure: [outcome, unit and reading rule]. Keep consistent: [factor and method for each control]. Record: [readings and any changes to the setup].” Read those lines as instructions and look for a decision someone would still have to guess.

If a factor cannot be controlled, say how you will observe or record it and what uncertainty it may introduce. Do not label it controlled simply because you want it to stay the same. The finished plan should make clear which comparison the experiment can support and which conditions remain uncertain.