What it does
A fair test is an experiment where only one thing is different between the tests, so any difference in the result must come from that one thing. This page helps you plan one. Pick a question, such as “Does adding salt make water boil sooner?”. For each thing that could affect the result (the amount of water, the flame, the pan, the lid), choose Change or Same. Then pick what you will measure. The picture shows Test A and Test B side by side and marks every line that differs: green when it is the one thing you meant to change, red when something else has crept in. A results table then lets you type your measurements for each repeat and works out the averages and the spread.
It follows the school topic Class 8 Science, exploring the investigative world of science (asking questions, fair tests, observing and measuring), and it is useful in every later class that has practical work. After planning, write it up with the lab report, and when you have results, the uncertainty propagation calculator shows how sure you can be about them.
How to use it
- Choose a question from the list.
- Tap Change next to the one thing that should differ between Test A and Test B. Leave everything else on Same.
- Choose what to measure. It must be the thing the question asks about: “sooner” means a time, “taller” means a height.
- Set the number of repeats. Three is a good habit.
- Read the verdict under the picture. Red lines say what to fix; green lines say what is right.
- Type your results in the table. The page shows each test’s average and spread, and says whether the difference is bigger than the spread.
The Level switch adds the words used from Class 9 on: the independent variable (what you change), the dependent variable (what you measure), the control variables (what you keep the same) and a hypothesis you could test.
Worked example
Question: “Does sugar dissolve faster in hot water?”. Set water temperature to Change (cold 15 °C in A, hot 60 °C in B), keep the amount of sugar, the amount of water, the stirring and the kind of sugar the same, and measure the time until no sugar is left. The page says “A fair test.”
Now also set stirring to Change. The verdict becomes “Not fair yet: 2 things change at once”, because if B dissolves faster you cannot tell whether the heat or the stirring did it.
Type results 42, 45, 40 seconds for A and 14, 16, 15 for B. The averages are 42.33 and 15 seconds, the spreads 5 and 2, and the difference of −27.3 seconds is far bigger than the spread, so it is likely to be real.
Check yourself
Five fresh questions: which thing to change, what to measure, whether a plan with two changes is fair, an average of three repeats, and why we repeat. Nothing is saved.
Tips
- Write down everything you keep the same. It is easy to forget the size of a pot or the time of day.
- Measure with numbers when you can (seconds, centimetres) rather than words like “fast”.
- One odd result? Look for a reason (a mistake, a draught) and repeat the test rather than quietly dropping it.
FAQ
Can a test ever change two things? Not if you want to know what caused the result. Scientists sometimes study several things at once with special designs, but each comparison still changes one thing at a time.
Why is the spread useful? If the difference between the averages is smaller than the spread between repeats, it might just be chance.
Is this from NCERT? No. It is an independent study aid that follows the same school topics, with its own investigations and numbers.
Where the numbers come from
- The fair test idea (change one variable, keep the others the same, measure the outcome, repeat and average): standard school science method. The investigations and numbers on the page are made up for practice.
- Checked by hand: every investigation gives a fair verdict only for the one right plan, and the averages match a calculator.
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Last updated 2026-11-09.