Science skills · Grades 5–8
Independent, Dependent & Controlled Variables: Examples
Variables help turn a science question into a test that produces interpretable evidence. Before changing anything, decide which factor you will deliberately change, what result you will measure and what conditions must stay comparable.
What are the three types of variables?
The independent variable is the factor the investigator intentionally changes. The dependent variable is the outcome measured in response. Controlled variables (constants) are the conditions deliberately kept alike so the comparison remains fair.
For example, if the question asks whether different sunlight durations affect plant height, sunlight duration is the independent variable. Height, measured in centimetres after the same growing period, is the dependent variable. Plant species, soil, pot size and water should be held constant.
A practical method for identifying variables
First underline the exact question being tested. Ask 'What changes on purpose?' and label that input. Ask 'What number or observation will show the result?' and label that output. Finally list other conditions that could affect the result; choose which of those must be controlled.
A controlled variable is not the same as a control group. A control group is a comparison group, while a controlled variable is a condition kept consistent between groups. Some investigations use both; others need no separate untreated group.
How would a teacher assess the skill?
For Grades 5–6, start with familiar materials and identify the changed and measured factors. For Grades 7–8, also describe measurement units, a repeatable procedure and at least two controls. Ask students to justify how controlling them makes a comparison more credible.
Do not confuse correlation with proof of causation. If water and light both change between the groups, the outcome cannot be confidently attributed to sunlight alone.
Worked example: sunlight and seedlings
Two groups of the same bean seedlings receive four or eight hours of sunlight per day. Each group receives 100 mL of water daily, uses matching pots and soil, and grows for 14 days. Students record each seedling's height in centimetres.
Question
Identify the independent variable, dependent variable and two controlled variables.
Explanation
Independent: hours of sunlight per day. Dependent: seedling height after 14 days (cm). Controls: water volume per day and plant variety; pot size and soil type should also be held consistent.
Measure several seedlings in each group and repeat the conditions before deciding whether any difference is reliable.
Check your understanding
Students test whether adding more salt changes the time ice takes to melt. What factor do they change, what do they measure, and what should they keep the same?
Reveal the suggested answer
Change: amount of salt. Measure: melting time, using the same timing rule. Keep the same: initial ice mass, ice shape and starting temperature. Repeating trials helps assess consistency.
Frequently asked questions
Is the dependent variable always a number?
Not always; it can be a systematic observation. A measurable quantity with units is often easier to compare and interpret.
Can an experiment have more than one independent variable?
Yes, advanced designs can vary multiple factors, but beginners should usually change one factor at a time to understand fair comparisons.
Apply what you learned
Use a new science scenario to test your understanding. Adjust grade, difficulty and topic; no account is required.