AC9S7I02 · YEAR 7 · INQUIRY

Planning a Fair Test

ACARA v9 CONTENT DESCRIPTION “plan and conduct reproducible investigations to answer questions and test hypotheses, including identifying variables and assumptions and, as appropriate, recognising and managing risks, considering ethical issues and recognising key considerations regarding heritage sites and artefacts on Country/Place”
Builds on framing a testable question. Once you know the one thing to change, a fair test makes the result trustworthy: you hold every other variable the same so the change you see can only be caused by the variable you chose.

What makes a test fair

A fair test changes one variable, the independent variable, and keeps everything else the same. The things you keep the same are called controlled variables. If two things change at once, you can never tell which one caused the result. Holding the rest steady is what turns a rough trial into reliable evidence.

Make the plant test fair
You are testing how the colour of light affects how tall bean seedlings grow. The light colour is the one thing you change. Decide what to do with every other variable.
Three trays of bean seedlings sit under red, blue and white grow lamps. You want to find out which light colour grows the tallest plants over two weeks.
Variable being tested: The colour of the grow light (red, blue or white) (this one we change)
Amount of water each tray gets per day
Size and type of pot, and the same potting soil
Room temperature and hours of light per day
Type and starting size of the bean seedlings
Not a fair test yet: more than one thing is changing, so you could not tell which change caused the result. Hold every other variable the same.

Reproducible means others can repeat it

Planning also means writing the method clearly enough that someone else could follow it and get a similar result. That includes how much water, how many hours of light, how you measure height, and growing several seedlings under each colour so one unusual plant does not mislead you. Repeating and averaging makes the evidence stronger.

Plan the safety and repeats
A good plan also manages risk and builds in repeats. Confirm the steps that keep the test safe and reliable.
Before you start, you check how to run the investigation safely and how to make the result reliable.
Variable being tested: The colour of the grow light (red, blue or white) (this one we change)
Wash hands after handling soil and wipe up water spills
Switch lamps off when leaving so they do not overheat
Grow three seedlings under each colour and average their heights
Not a fair test yet: more than one thing is changing, so you could not tell which change caused the result. Hold every other variable the same.

Is this plan actually reproducible?

Before running the test, a good scientist checks the written plan against the idea of a fair, repeatable investigation. The claim is that the plan is reproducible: another class could follow it and reach the same conclusion. Sort each part of the method by whether it genuinely supports that claim, or whether it would undermine reproducibility instead.

Judge the plan against fair-test rules
The claim: this written plan is a reproducible fair test. Decide which method choices support that claim.
Claim: This plan is a reproducible fair test of how light colour affects seedling height.
Each tray gets exactly 50 mL of water at the same time every day.
The method states the lamp colours used and the distance from lamp to tray.
Whoever waters the plants decides the amount by eye, differently each day.
Three seedlings grow under each colour and their heights are averaged.
The blue lamp tray is moved nearer a sunny window than the others.
Decide whether each statement is evidence for the claim, or not.

What the reproducible test produced

Run carefully, the fair test yields data worth graphing. After two weeks, the average height of the seedlings under each light colour is recorded. Because only the colour was changed and everything else held the same, the differences in height can be read as the effect of the light colour itself.

Average seedling height after two weeks
The average height of three bean seedlings grown under each light colour. Switch between table and bar chart to compare the colours.
Holding water, pot, soil and temperature the same means the height differences trace back to the light. Blue light grew the tallest seedlings and the seedlings left in the dark barely grew, the result a fair, repeatable plan was designed to reveal.

Why this matters

Scientists trust results that come from fair, repeatable tests. By changing one variable, controlling the rest, managing risks and writing a clear method, you produce evidence others can check. This is the backbone of every reliable experiment, from a school garden to a medical trial.

Quick self-check
1. In a fair test, how many variables should you deliberately change at a time?
2. You are testing how light colour affects how tall seedlings grow. Which must be kept the same?
3. An investigation is reproducible when...
4. Why might you grow three seedlings under each light colour instead of one?
5. Which is a sensible safety step when running this plant investigation?