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Science · Original worked example

Separate a control group from controlled variables

Identify what an experiment changes, measures, and holds constant. Distinguish a comparison group from controlled conditions in original science examples.

By Raise Your ACT · Updated October 27, 2026 · How we review content

Before you begin

Useful foundation: Read an experiment's setup and distinguish a treatment from a measurement.

Your goal: Identify a useful comparison and a change that would make a result harder to interpret.

The independent variable is the factor deliberately changed. The dependent variable is the measured response. Controlled variables are conditions kept consistent. A control group provides a comparison, often without the treatment being tested.

Work through an example

Students test whether adding fertilizer changes seedling growth. They randomly assign 40 similar seedlings to two groups. Both receive equal water and light in the same soil. One group receives fertilizer; the other receives none. Height gain is measured after three weeks.
  1. The deliberate change is whether fertilizer is added. That is the independent variable in this setup.
  2. The response is height gain after three weeks, not simply final height. That is the dependent variable.
  3. Water, light, soil, and measurement interval are kept consistent. These are controlled conditions.
  4. The group receiving no fertilizer provides a comparison for growth without the added treatment. Random assignment and multiple seedlings help address natural variation, but do not remove every possible source of error.

Try a question yourself

Researchers compare how quickly equal masses of sugar dissolve in water at 20°C and 40°C. They use the same sugar type, water volume, containers, and stirring rate, and measure the time until no crystals remain visible. This is an original illustrative setup.
Which change would make it harder to isolate the effect of water temperature on dissolving time?

Choose an answer, then check your reasoning. This public practice is not saved.

Read all answer explanations

Repeating each temperature condition several times.: Repeated trials help reveal variation. They do not, by themselves, introduce a second systematic difference between temperatures.

Using the same water volume in both conditions.: Equal water volumes keep one potentially relevant condition consistent, helping isolate the temperature difference.

Using the same rule for when to stop the timer.: A consistent endpoint makes the time measurements more comparable across the two conditions.

Stirring the 40°C water faster than the 20°C water. — correct answer: Now both temperature and stirring rate differ. A time difference could reflect either factor or both, so the temperature effect cannot be isolated from this comparison.

What to do next

For each new experiment, write four short labels: changed, measured, kept consistent, and comparison. Then look for a competing explanation.

Revisit the idea in a week using a different question. Repeating this example can help you understand it, but remembering its answer is not evidence of learning a new problem.

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These original practice examples are independent of ACT and are not official test questions.