Skip to content
sciencelab
0
0
0 XP
Lvl 1
KJ
Lesson 3 ~35 min Unit 4 · Data Science +85 XP

Variables and Fair Testing

In 2022, Australian scientist Dr Sandy Hoffmann grew 60 tomato plants under identical light and water — changing only nitrogen dose — to find the exact level that doubled yield.

Today's hook: In 2022, a team at the University of Queensland tested whether nitrogen fertilizer improves tomato growth. They planted 60 seedlings in identical soil, under the same lights, given the same water — but split into 3 groups receiving 0 g, 2 g, or 4 g of nitrogen per litre. Only by keeping everything else the same could they be sure nitrogen was the cause of any difference. If they had also changed the watering schedule, could they still trust the result? What would be different about their conclusion?
0/5QUESTS

Printable Worksheets

Print or save as PDF — or build a custom worksheet from any module's questions.

Build Apply Master Build custom
Think First
warm-up

A student wants to find out whether adding salt makes water boil faster. They boil plain water on high heat and salty water on low heat.

What is wrong with this experiment? Why can the student not trust their conclusion about salt?

Write your prediction in your book before reading on.
1
What is a Variable?
+5 XP

Imagine two pots of tomatoes on the same windowsill. You add fertilizer to one but also give it more water, put it closer to the glass, and use a larger pot. At the end of the month, it grows 15 cm taller. Did the fertilizer cause that? You cannot say — too many things changed at once. A variable is any factor in an experiment that can change, such as temperature, time, amount of substance, or light intensity. In every well-designed experiment, variables play one of three roles. The independent variable is the factor you deliberately change. The dependent variable is the factor you measure to see what happens. All other important factors become controlled variables — they are kept identical across every trial so they cannot confuse your results.

Understanding these roles is essential for designing valid investigations. If you do not know which variable is which, you cannot tell whether your results actually answer your question. A common beginner mistake is to change more than one thing at a time, which makes it impossible to know what caused any observed effect.

The Three Types of Variables in an Experiment INDEPENDENT VARIABLE (IV) What you CHANGE You control this e.g. amount of light given to plant → plotted on x-axis DEPENDENT VARIABLE (DV) What you MEASURE Responds to IV e.g. plant height after 2 weeks → plotted on y-axis CONTROLLED VARIABLES (CV) What you KEEP SAME Ensures fair test e.g. water, soil type pot size, temperature → listed in method A fair test changes only the IV — all other conditions (CVs) stay the same
Example

A student wants to know how light affects plant growth. The amount of light is the independent variable (they change it). The height of the plant is the dependent variable (they measure it). The type of plant, amount of water, soil type, and pot size are all controlled variables (kept the same in every group).

Real-world anchor

CSIRO agricultural researchers run field trials across Australia to test new crop varieties. In every trial, they control soil moisture, planting depth, and pest management so that only the crop variety (the independent variable) differs. This discipline is why their recommendations are trusted by farmers nationwide.

Watch out

Many students think that anything in an experiment is a variable. This is not true. Controlled variables are deliberately prevented from changing. They are still variables in principle — they could change — but in a fair test they are held constant so they do not interfere with the relationship you are studying.

Which best describes a variable?
2
What You'll Master
objectives

Know

  • The independent variable is the factor deliberately changed by the investigator.
  • The dependent variable is the factor measured or observed to see the effect.

Understand

  • A fair test changes only the independent variable while keeping all other conditions the same.
  • Controlled variables must be held constant so they do not affect the results.

Can Do

  • Identify the independent, dependent and controlled variables in an experiment.
  • Suggest improvements to make an investigation a fair test.
Cross-lesson links: The variables you identify here are exactly what go into the data table you design in Lesson 4 (Designing Reliable Data Tables), and they reappear in Lesson 11 (Claims, Evidence and Reasoning) when you judge whether evidence truly supports a claim.
3
Words You Need
vocabulary
VariableAny factor, condition or quantity that can change in an experiment.
Independent variableThe factor that is deliberately changed by the investigator to test its effect.
Dependent variableThe factor that is measured or observed to see how it responds to changes in the independent variable.
Controlled variableA factor that is kept constant so it does not influence the outcome of the experiment.
Fair testAn investigation in which only the independent variable is changed, while all other variables are kept the same.
ValidityThe extent to which an experiment actually tests what it claims to test.
Match each term to its definition.
  • Independent variable
  • Dependent variable
  • Controlled variable
  • Fair test
  • Only one variable is changed at a time
  • A factor kept constant
  • The factor measured or observed
  • The factor deliberately changed
4
Spot the Trap
heads-up

Wrong: A fair test means being nice to everyone doing the experiment.

Right: In science, a fair test means controlling variables so that only the factor being tested can affect the results.

Wrong: You should change as many variables as possible to save time.

Right: Changing multiple variables at once makes it impossible to know which one caused the effect. Only change one independent variable at a time.

Wrong: Confusing the independent and dependent variables.

Right: The independent variable is what YOU change, and the dependent variable is what you MEASURE. A helpful phrase is 'I change the independent to see what happens to the dependent'.

Wrong: Forgetting to list obvious controlled variables.

Right: Always think broadly about what could affect the result. Temperature, time, amount of material, equipment and location are common controlled variables.

Spot the slip-up+5 XP

A student's experimental plan has an error — click it.

A student wants to test whether fertiliser makes plants grow taller. Their plan:
  1. I will use the same type of plant and pot for every group.
  2. I will give one group fertiliser and the other group nothing.
  3. I will put the fertilised plants in full sun and the others in shade.
5
The Independent Variable
+5 XP

The independent variable is the factor you choose to change in order to test its effect. It is the 'cause' you are investigating. A good experiment has only one independent variable because that makes it possible to link any observed effect directly to the factor you changed.

Choosing the right independent variable requires a clear research question. If your question is 'Does temperature affect solubility?' then temperature is your independent variable. If your question is 'Which brand of battery lasts longest?' then battery brand is your independent variable. Everything else must be controlled.

A Fair Test — Only One Variable Changes UNFAIR TEST Plant A: more light AND more water Plant B: less light AND less water Cannot tell which factor caused difference → FAIR TEST Plant A: more light, same water Plant B: less light, same water Light is the IV — can isolate its effect Only changing one variable lets you draw a valid conclusion about cause and effect
Example

A student wants to know how exercise affects heart rate. The type of exercise — walking, jogging, sprinting — is the independent variable. The student deliberately changes this factor and then measures the effect on heart rate. They do not also change the time of day or the room temperature, because those would become extra independent variables.

Real-world anchor

At ANSTO, scientists testing radiation shielding materials change only the shielding type (the independent variable) while keeping radiation source, distance, and duration identical. This controlled approach ensures that any difference in protection is caused by the material itself, not by other factors.

Watch out

Some students try to change several things at once to 'save time.' This destroys the validity of the experiment. If you change both fertiliser amount and light exposure, and the plants grow differently, you have no way of knowing which change caused the result — or whether they interacted in unexpected ways.

In an experiment to test how exercise affects heart rate, what is the independent variable?
6
The Dependent Variable
+5 XP

The dependent variable is the factor you measure or observe to see how it responds. It is the 'effect' you are looking for. The value of the dependent variable depends on the independent variable — that is why it is called 'dependent.'

Choosing what to measure is a critical scientific decision. The dependent variable must be something you can record reliably and repeatedly. Vague measures like 'how happy the plant looks' are weak. Precise measures like 'plant height in centimetres' or 'number of leaves' are strong because they can be verified by another observer.

Identifying Variables in an Experiment "We tested how salt concentration affects the time it takes water to boil." IV (change): salt concentration DV (measure): time to boil (seconds) CVs (keep same): volume, heat source, pot Look for: what changes, what is measured, what stays constant
Example

In the plant growth experiment, the dependent variable could be the height of the plant measured weekly with a ruler. Alternatively, it could be the number of new leaves, the total mass of the plant, or the rate of photosynthesis. The choice depends on what the researcher wants to know and what tools they have available.

Real-world anchor

The Bureau of Meteorology studies how El Niño affects Australian rainfall. El Niño conditions are the independent variable; rainfall totals across regions are the dependent variable. By measuring rainfall precisely over decades, meteorologists can predict drought risk with increasing confidence.

Watch out

A common error is confusing the dependent variable with the controlled variable. The dependent variable is what you measure, not what you control. You control water amount; you measure plant height. If you say you are 'controlling plant height,' you have reversed the roles and your experiment will not work.

True or false?
The dependent variable is the factor the scientist deliberately changes.
7
Controlled Variables and Fair Testing
+5 XP

Controlled variables are all other factors that could affect the outcome. To keep the test fair, these must stay the same across all groups. If even one controlled variable drifts — different soil in one pot, more water given by accident — your results become unreliable because you can no longer be sure what caused the difference.

In practice, controlling variables means making deliberate choices before the experiment begins. You decide on one type of plant, one brand of soil, one size of pot, one watering schedule. You write these decisions down in your method so another scientist can replicate your exact setup. Replication is impossible if your controls were accidental rather than planned.

Controlled Variables Make Conclusions Valid If a CV is NOT controlled: You cannot be sure which variable caused the result e.g. if one plant gets more water AND more light you cannot conclude that light caused the extra growth CVs = the "all else equal" that science depends on
Example

In a fertiliser experiment, you would keep the type of plant, amount of water, soil type, pot size, and room temperature identical for every plant. Only the amount of fertiliser would differ. If one plant gets more sunlight because it is nearer a window, sunlight becomes an uncontrolled variable and your results are meaningless.

Real-world anchor

CSIRO runs long-term ecological monitoring sites where every variable is controlled except the one being studied. At their alpine research plots, temperature sensors, soil samplers, and vegetation surveys follow identical protocols across decades. This strict control is why their climate-impact data is trusted by policymakers.

Watch out

Some students think that if a result seems 'obvious,' you do not need controls. This is dangerous thinking. What seems obvious can be wrong. Before scientists proved that stomach ulcers were caused by bacteria, everyone 'knew' they were caused by stress. Without controlled experiments, false 'obvious' conclusions persist for generations.

Interactive cycle+7 XP

Click each stage to see what happens in a fair test.

Identify

State your question clearly and pick the single factor you want to test. This becomes your independent variable.

Speed Round · 6 questions

Speed round +6 XP

True or false? Tap as fast as you can. Build a streak.

Q · 1 / 6 Streak · 0 Score · 0

A variable is any factor that can change in an experiment.

How are you completing this lesson?

Revisit Your Thinking
reflect

At the start of the lesson you were asked: "I added fertilizer and the plant grew — was the fertilizer the cause?" Before the lesson, you might have said "yes" just because the two things happened together.

Now that you understand variables and fair testing, how has your answer changed? What would you need to control to actually prove the fertilizer caused the growth — and how is that different from just observing that growth happened?

Redesign the experiment so it is a fair test, clearly identifying the independent, dependent and controlled variables.

Write your updated thinking in your book.

Quick Check · 5 questions

1
In an experiment to test how exercise affects heart rate, what is the independent variable?
+10 XP
2
Which best describes a controlled variable?
+10 XP
3
Why is it important to have only one independent variable in a fair test?
+10 XP
4
A student tests different brands of batteries by putting them in different devices. What is wrong with this test?
+10 XP
5
Which statement about validity is correct?
+10 XP

Check Your Understanding · 3 questions

Check Your Understanding
short answer

1. In an experiment testing whether fertiliser helps plants grow, identify the independent, dependent and one controlled variable.

Write your answer in your book.

2. Explain why changing the temperature and the amount of sugar at the same time makes an experiment about dissolving unfair.

Write your answer in your book.

3. What would happen to the validity of an experiment if the controlled variables were not kept constant?

Write your answer in your book.

Show Your Working · 3 questions

Show Your Working
12 marks total
5 MARKS

SA1. Define independent, dependent and controlled variables, and explain how each contributes to a fair test.

Write your answer in your book.
4 MARKS

SA2. A student wants to find out if the colour of a cup affects how quickly coffee cools. Identify the variables and describe how to conduct a fair test.

Hint: Think about what must stay the same and what will be measured.

Write your answer in your book.
3 MARKS

SA3. Explain what would happen to the reliability of an experiment if two independent variables were changed at the same time.

Write your answer in your book.
Comprehensive Answers

Quick Check

1. B — Type of exercise is the independent variable.

2. C — A controlled variable is kept constant to ensure fairness.

3. B — Only one independent variable lets you identify which factor caused the effect.

4. B — The device is a controlled variable that is not kept constant.

5. B — A valid experiment tests what it claims to test.

Show Your Working Model Answers

SA1 (5 marks): Independent variable: the factor deliberately changed by the investigator [1]. Dependent variable: the factor measured or observed to see the effect [1]. Controlled variable: a factor kept constant so it does not influence the outcome [1]. Only changing the independent variable allows you to identify cause and effect [1]. Keeping controlled variables constant ensures any change in the dependent variable is due to the independent variable [1].

SA2 (4 marks): Independent variable: colour of the cup [1]. Dependent variable: time taken for coffee to cool / temperature after set time [1]. Controlled variables: volume of coffee, starting temperature, room temperature, cup material and thickness, time [1]. Use identical cups in different colours, same volume and starting temperature, measure temperature at regular intervals [1].

SA3 (3 marks): Reliability would be affected because you cannot tell which variable caused the effect [1]. The results would be difficult to replicate consistently [1]. Changing multiple variables introduces extra factors that make the data untrustworthy [1].

R
Quick Review

Variable

Any factor that can change in an experiment

Independent

The factor deliberately changed

Dependent

The factor measured or observed

Controlled

Kept constant so it does not influence results

Fair test

Only the independent variable is changed

Validity

The experiment tests what it claims to test

Test Your Knowledge
+25 XP

Put what you have learned to the test! Jump through the questions in game form.

Play Game

Your Badges

0 of 6
First Steps
3-Day Streak
3 in a Row
Lesson Ace
Stretch Seeker
Daily Warrior

Mark lesson as complete

Tick when you've finished Learn, Practice and the game. Earns +85 XP and +25 coins.

Unit overview · Science subject page · Unit quiz

🎓
Want help with Lesson 3 — Variables and Fair Testing?

Work through this topic 1-on-1 with an experienced HSC tutor.

Book a free session →