Lessons · Teaching · the fair test, and the three kinds of variable
One thing changed, one thing measured, everything else held still
In a fair test the independent variable is the one thing you deliberately change, the dependent variable is the one thing you measure, and the control variables are everything else, held the same. Change two things at once and no result can tell you which caused what.
Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.
In beta. This lesson was written for Hone and has not yet been checked by a state certification officer. Practice material, not professional advice. What that means.
What it is for
This is the most examined idea in elementary science and the most useful thing a pupil takes out of the subject, because it is how any claim is checked for the rest of their life. It is also what makes a classroom investigation an investigation rather than an activity.
How to think about it
Before touching anything, write the three lists: what I will change, what I will measure, and what I must keep the same. The third list is the long one and the one that gets skipped. Then say what result would show the idea is wrong, because a test that cannot fail is not a test.
Worked example
Independent: the one thing changed, such as the amount of light a plant getsOne. Changing two makes the result unreadable.
Dependent: the one thing measured, such as the height after two weeksIt depends on what you changed, which is where the name comes from.
Control: water, soil, pot size, temperature, the kind of seed, all held the sameThe longest list, and the one an examination asks you to complete.
Repeat the test, because one result could be chanceRepeats are what separate a result from an anecdote.
Your turn
Name the variable that is deliberately changed in a fair test.
The variable is the one you change
Solve one, graded on the server
The trap
Calling everything you did not change a control variable while leaving out the ones you did not think of. The ones nobody listed are exactly the ones that ruin the result.