Fair tests: variables, repeats and reliable results

Three kinds of thing in every test

The thing you change on purpose; the thing you measure; and the things you keep the same. Writing these three lists before you start is the whole of planning at this stage, and a Checkpoint question about a fair test is always asking for one of the three.

Why keep things the same

If two things change at once, you cannot tell which one caused the result. That is not a rule to memorise — it is the reason the whole method exists.

Repeats and odd results

Measure each thing at least twice. If one number is far from the others, it is an anomalous result: check it or measure again rather than quietly including it in an average.

Making measuring fairer

Use the same instrument, read it the same way, and start from zero each time. Small habits like these are what turn a rough test into evidence.