How to answer it
- Find what the experimenter changed on purpose: the independent variable.
- Find what was measured: the dependent variable.
- List what was kept the same: the controls.
Worked example
An original question in the ACT format, already answered.
ACT ScienceWorked example
A student tests whether fertilizer affects plant height. She gives four groups of the same plant species 0, 5, 10 and 15 grams of fertilizer and keeps light, water and soil the same.
Which is the independent variable?
- APlant height
- BAmount of fertilizer
- CAmount of light
- DSoil type
Answer: B. The student changed the fertilizer amount on purpose, so it’s the independent variable. Height is measured, and light and soil are held constant.
The most common trap
Choosing what was measured. The independent variable is what the experimenter changes.
Practice it
Hourly Prep’s daily sets mix scientific investigation with the rest of ACT Science, and send more of it your way when it’s a weak spot. See how the ACT prep plan works, or browse the other ACT skill guides.