MYP Biology: Controlling Variables in Biology Investigations

Controlling variables means keeping every factor except the one you're testing (the independent variable) exactly the same, so any change in your dependent variable can only be explained by that one factor. In MYP 4-5, this skill sits directly under Criterion B: Inquiring and designing.

The independent variable is the one factor you deliberately change; the dependent variable is what you measure as a result; controlled variables are everything else you keep constant so they can't interfere. Confuse these three and your experiment stops testing what you think it's testing.

If you don't control other variables, you can't be sure your independent variable caused the change you observed — a rival factor, called a confounding variable, could be responsible instead. Examiners call this validity, and it's one of the first things I check when marking a student's method.

A fair test is an experiment where only the independent variable changes between trials — every controlled variable stays fixed, and you repeat trials enough times to check reliability. A genuinely fair test needs a stated range for the independent variable and named values for every controlled one.

Start by naming your independent and dependent variable, then ask: what else, if it changed, could affect my dependent variable? List every factor you can think of, decide a realistic way to control each one, and write the exact value or method into your plan before you touch any equipment.

In a pondweed photosynthesis experiment testing light intensity, your controlled variables should include water temperature (fixed at 20°C with a water bath), carbon dioxide concentration (a set amount of sodium bicarbonate), and identical pondweed species and stem length across every trial — otherwise you can't tell what actually caused any change.

The three I see most often: naming a controlled variable without saying how it's controlled, listing so many variables the method becomes impossible to actually carry out, and mixing up a controlled variable with the dependent variable. Any one of these costs marks against Criterion B strand ii.

Criterion B, Inquiring and designing, is marked out of 8 in MYP 4-5, and one strand specifically asks you to describe how you will manipulate variables and collect relevant data. Naming your controlled variables with real values is exactly what that strand rewards — vague answers score low even with strong ideas.

For controlling variables you'll usually meet "state" (name the variable), "describe" (give enough detail that someone else could repeat it), or "explain" (say why controlling it matters for validity). According to the MYP command terms list, "describe" requires a detailed account — a single word like "temperature" is never enough on its own.

In MYP 4-5, controlling variables is marked under Criterion B as part of designing your own investigation, worth up to 8 marks. In DP Biology, the same skill reappears in the Internal Assessment under the Exploration criterion and, since the current guide's first exams in 2025, in exam-based data-analysis questions too.

Good results don't rescue a vague method. Examiners mark what you planned and controlled, not just the numbers you produced — so a student with a clean graph but no stated method for keeping pH or temperature constant still loses marks, because the design itself was never shown to be valid.

You don't need a science background to help with this. Ask your child to explain, in one sentence per variable, exactly what they changed, what they measured, and what they kept the same — if they stumble on any of the three, that's the gap to close before their next mock or assessed investigation.

Look for materials built around real investigation scenarios — enzymes, photosynthesis, osmosis, respiration — rather than abstract definitions of the three variable types. Students learn to identify and justify controlled variables by practising on actual biology contexts repeatedly, and by having a marked worked example to compare their answer against.