5.1 Variables & Fair Tests

Every science investigation is a careful argument. You change one thing, watch what happens to something else, and keep absolutely everything else identical so the result means something. ICAS Science tests whether you understand that argument - not whether you have memorised facts - so learning to name and control variables (the things that can change or be changed in an experiment) is the foundation of the whole Investigating skill area.

The Three Kinds of Variable

• The independent variable is the one thing the investigator deliberately changes on purpose. You choose it before the experiment starts.
• The dependent variable is the thing you measure or observe - it depends on what you changed. It is your result.
• The controlled variables are all the other things you keep exactly the same in every trial, so they cannot interfere.

A quick memory trick: I change the independent variable, I measure the dependent variable, and I keep the controlled variables constant.

Four Worked Examples

These four experiments appear in school labs all over Australia, and ICAS stimulus questions are built from ones just like them. For each, practise naming all three kinds of variable.

Notice how long the controlled-variables column is. That is the point: a good experiment controls many things so that only one thing can explain the result. In the dissolving experiment, for example, if one cup had bigger sugar grains, the grains - not the temperature - might be the real reason the sugar dissolved faster.

Change Only ONE Thing at a Time

This is the golden rule of a fair test (an experiment where only the independent variable differs between setups). Suppose Liam wants to know whether sunlight affects plant growth, so he puts one plant on a sunny windowsill and one in a dark cupboard - but he waters the windowsill plant every day and forgets the cupboard plant for a week. When the windowsill plant grows taller, what caused it? The light, or the water? Nobody can say, because two variables changed at once. The test is unfair and the result tells us nothing. Whenever an ICAS question describes an experiment, your first job is to count how many things differ between the setups. If the answer is more than one, the experiment cannot answer its question.

The Question an Experiment Answers

Every fair test answers a question shaped like this: "How does changing X affect Y?" where X is the independent variable and Y is the dependent variable. ICAS often flips this around: it describes a method and asks which question the experiment was designed to answer. The trick is to look at what the student actually changed and what they actually measured - ignore anything they merely kept the same.

For example, if a student rolls the same toy car down ramps that are 10 cm, 20 cm and 30 cm high and measures how far the car travels each time, the experiment answers "How does the height of a ramp affect the distance a toy car rolls?" It does not answer questions about the car's mass or the ramp's surface, because those were never changed.

Spot the Unfair Test: The Number One ICAS Question

The most common investigating question on the paper describes an experiment with a flaw and asks which variable was not controlled, or what the student should keep the same. These questions are easy marks once you train yourself to hunt for the sneaky second difference.

The unfair-test checklist

Run through this list every time you read a described experiment:

1. Was only one thing changed between the setups or groups? Name it.
2. Same materials? Same brand of battery, same type of seed, same kind of liquid, same size of container.
3. Same amounts? Same volumes, masses, lengths and numbers of items.
4. Same equipment and method? Same thermometer, same ruler, same way of timing.
5. Same conditions and timing? Same temperature, light, location, and measurements taken after the same waiting time.
6. Any accidental second difference? One plant in sun and one in shade plus different watering; one car released from the top and one pushed.

A worked example

A class tests whether salt water freezes faster than fresh water. Beaker A holds 200 mL of salty water and Beaker B holds 100 mL of fresh water, and both go into the same freezer. Which variable was not controlled? The volume of water - a smaller amount of any liquid freezes faster, so if Beaker B freezes first, the result proves nothing about salt. The fix is to give both beakers 200 mL, keeping every condition identical except the salt.

When you practise past ICAS papers, say the three variables out loud for every experiment you meet: they changed this, they measured that, they should have kept these the same. It soon becomes automatic - and automatic is exactly what you want on exam day.