ACT Science Experimental Design: Variables, Controls & Research Summaries

Experimental-design questions are part of ACT Science's Scientific Investigation category.

ACT currently says Scientific Investigation makes up roughly 18–32% of the Science section. If you're still deciding how much to prioritize this section, see our guide on whether you should take ACT Science.

Independent variable

The factor deliberately changed.

Example:

Researchers grow plants under:

• 4 hours of light;
• 8 hours;
• 12 hours.

Independent variable:

hours of light

Dependent variable

The outcome measured.

If researchers measure plant height:

Dependent variable:

plant height

Control variables

Things kept constant.

Examples:

• soil type;
• pot size;
• water amount;
• plant species.

This helps isolate the effect of the independent variable.

Control group

A comparison group that does not receive the experimental treatment, when appropriate.

Example:

Treatment group receives fertilizer.

Control group receives no fertilizer.

Repeated trials

Repeating an experiment can help researchers see whether the result is consistent rather than a one-time accident.

Research summaries

ACT Science often gives you:

• experiment setup;
• table or graph;
• several trials;
• questions about what changed.

Do not memorize the entire passage. This approach applies to Data Representation and Research Summaries passages; Conflicting Viewpoints passages need a different approach, covered in our guide to ACT Science conflicting viewpoints.

Map:

1. what was changed;
2. what was measured;
3. what stayed constant;
4. what happened.

Predicting a new trial

Suppose:

If the trend continues, the reaction time at 40°C is likely:

• lower than 8 s.

You do not need the exact value unless the data support one.

Improving an experiment

A question may ask how to test a new idea.

Example:

Current experiment changes temperature.

New question: Does salt concentration affect reaction time?

Best modification: keep temperature controlled and vary salt concentration.

Original practice

1

Plants receive different amounts of fertilizer. Researchers measure leaf count.

Independent variable?

2

Same experiment.

Dependent variable?

3

Why keep the plant species constant?

A. to increase the independent variable
B. to reduce another source of variation
C. to eliminate the dependent variable
D. to guarantee the hypothesis is correct

4

Why repeat a trial multiple times?

Answers

1. fertilizer amount
2. leaf count
3. B
4. to check whether the result is consistent/reliable

No calculator

ACT does not permit a calculator on Science.

Most calculations are simple enough to do mentally or by hand. For a complete overview of the current test format, see our Enhanced ACT guide, and the rest of our ACT guides for other sections.

Official sources

• ACT Science Test Description: https://www.act.org/content/act/en/products-and-services/the-act/test-preparation/description-of-science-test.html
• ACT Enhanced FAQs: https://www.act.org/content/act/en/products-and-services/the-act-educator/the-act-test/enhancements-k12/faqs.html

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