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Biology · Key Stage 3 · The School
One thing at a time
A fair test changes ONE variable, measures one outcome, and keeps everything else the same. Testing whether light affects plant growth means identical pots, identical soil, identical water, identical seeds — and only the light different. Change two things and you cannot say which one caused the result, however striking the result looks.
Living things vary, so one is never enough
Two plants given identical treatment will still grow differently, because organisms vary. That is why biology repeats: grow ten plants per condition, not one, and compare the MEANS. A single dramatic plant proves nothing; the spread of ten tells you whether the difference is real or just variation.
Sampling and identifying
You cannot count every daisy on a field, so you sample: throw a QUADRAT at random, count what is inside, repeat, and scale up. Random placement matters — choosing where to put it biases the result toward whatever caught your eye. To name what you find, use a DICHOTOMOUS KEY: a series of two-way questions that narrows the possibilities each time.
Two seedlings, one fed and one not. The unfed one is taller. Have you shown that fertiliser stunts growth? No — individual plants vary enormously, and you have a sample of one in each group. Repeat with ten in each group and take the MEAN height: fed 18.2 cm, unfed 12.6 cm. Now you have something, because one unusually vigorous plant can no longer move the answer on its own.