Evidence supports a claim when it shows the pattern claimed, from a fair test, within the range tested. Ask three questions: does the data show this, was the test fair, and does the claim stay inside what was tested?
The three questions
- Does the data show it? Quote two numbers that match the claim.
- Was the test fair? Only the independent variable should have changed.
- Does the claim go further than the data? Check the range, the material and the wording.
If the answer to the third question is yes, the claim is an overreach, even when the first two pass.
A worked example
A student grows bean seedlings with 0 g, 10 g, 20 g and 30 g of fertiliser per pot. After two weeks the heights are 8 cm, 12 cm, 15 cm and 17 cm. The student claims: “More fertiliser always makes plants grow taller.”
The data shows height rising with each step, so the first question passes. The pots were the same size, with the same light and water, so the test was fair. The third question fails on two counts: only bean seedlings were tested, and nothing above 30 g was tried.
A claim the evidence does support
A limited version is: “For bean seedlings, height after two weeks increased as fertiliser rose from 0 g to 30 g.” Every word of it is covered by the table.
The gains also shrink: +4 cm, then +3 cm, then +2 cm. The benefit seems to be slowing, so any prediction for 40 g would be a guess.
The overreach trap
Some students want to sound confident, so they write “always”, “proves” or “all”. A conclusion earns marks for describing the relationship the evidence shows, not for being the widest statement.
Try putting “in this experiment” at the start of your conclusion. It prompts you to keep the claim to what was tested.
Try it yourself
A class heats 50 cm³ of water in three beakers with 1, 2 and 3 candles and records temperature rises of 12 °C, 25 °C and 36 °C in 5 minutes. A student says “Candles heat water faster than a Bunsen burner.” Does the evidence support this?
Answer
No. The data only compares 1, 2 and 3 candles, and the temperature rise grows with the number of candles.
A Bunsen burner was never tested, so there is nothing to compare it with. A supported claim is: with more candles, the water temperature rose more in 5 minutes. The original claim goes beyond what was measured.
Continue
Claims can also rest on hidden assumptions, which recognising an assumption covers. Go back to the scientific reasoning hub for the other skills, or try SPM Science tuition for one-to-one practice.