Variables & Fair Testing (Years 6–9) — Science

Independent, dependent and control variables, writing a testable hypothesis, and what makes an investigation a fair test.

Every real experiment starts with a simple but easy-to-get-wrong question: what exactly am I changing, what exactly am I measuring, and what must I keep the same so the result actually means something? Get this wrong and even a beautifully carried-out experiment produces useless data.

Why this matters Every practical exam paper marks you on identifying variables and designing a fair test — it is not optional extra content, it is core exam technique. It is also exactly how real scientists design real experiments: without a fair test, you cannot trust your conclusion, no matter how carefully you measured anything.

Independent, dependent and control variables

Take a classic investigation: does temperature affect how fast the enzyme amylase breaks down starch? To answer it scientifically, every variable in the experiment falls into one of three groups.

Independent variable (IV) — the one thing you deliberately choose to change. Here, that is the temperature of the water bath (e.g. 10°C, 25°C, 40°C, 55°C).
Dependent variable (DV) — the thing you measure as an outcome, to see whether it responds to the change in the IV. Here, that is the time taken for the iodine test to stop giving a blue-black colour (a proxy for how fast the starch has been digested).
Control variables (CV) — everything else that could also affect the result, which you deliberately keep the same every time: the volume and concentration of amylase, the volume and concentration of starch solution, the total reaction volume, and even who is timing and how the colour change is judged.

The independent variable is what you change (temperature); the dependent variable is what you measure (time); control variables are everything you keep the same.

Writing a testable hypothesis

A hypothesis is a clear, checkable prediction — not just a question. "Does temperature affect enzyme activity?" is a question. A proper hypothesis states the direction of the expected effect and, ideally, links it to a reason: "As temperature increases from 10°C to 40°C, the rate of starch digestion by amylase will increase, because the enzyme and substrate molecules will collide more often and more successfully — but above about 40°C the rate will fall sharply because the enzyme starts to denature." A good hypothesis can be shown to be right or wrong by the results; a vague question cannot.

What makes a test fair

A test is fair only when the independent variable is the one and only thing that changes between trials, and every control variable is kept genuinely constant. If two things change at once — say the temperature and the concentration of amylase — you cannot tell which one caused any difference in the result. An unfair test does not just give a slightly wrong answer; it gives a result that tells you nothing reliable at all, because you can no longer link cause to effect.

Sorting the variables of a pendulum investigation: one thing changed, one thing measured, everything else held constant.

Worked example. A student wants to find out how the length of a pendulum affects the time for one swing. She changes the string length AND uses a heavier bob for the longest string because "it looked more stable". Explain why this is not a fair test, and what she should do instead. Two variables changed at once — string length and bob mass — so if the swing time changes, she cannot tell whether length, mass, or both caused it. To fix this, she should keep the bob mass identical for every length tested, changing only the string length (the independent variable), while timing multiple swings and keeping the release angle the same each time.

Common mistake: writing a hypothesis as a question ("Does length affect time?") instead of a testable prediction with a direction ("As string length increases, the time period will increase, because..."). Also, students often list the dependent variable as a control variable by mistake — remember, the DV is the one thing you are not allowed to hold constant, because it is what you are measuring.

Memory hook: "I Deliberately Vary the Independent; the Dependent Does what it does and I measure it; everything else stays Constant." If you can't explain what you kept the same and why, the test isn't fair yet.