Study guide: Variation, controls, and scientific claims
Evaluate small comparisons using means, variation, and fair design.
Ideas to explain in your own words
- Identify a changed variable, measured outcome and limit of a small study.
- Use a range and overlapping results alongside group means.
- Explain why a design cannot isolate the tested factor.
- Specify a repeatable comparison and revise a claim with new results.
Watch your reasoning
- The supplied group means differ by 1 cm, but only six seedlings and one week were observed. The results do not show every plant doubling its growth, and water was held alike.
- The means are 4 and 3. Both groups vary, and values from their minimum–maximum spans overlap.
- Equal water removes the supplied water confound while preserving the intended light contrast. Other relevant conditions must also be kept alike.
- Both records matter. A failed repeat does not prove a factor is irrelevant in every setting, just as one initial gap is not a universal rule.
A worked review example
Make a fair plant comparison: The 6-hour mean is (2 + 3 + 4) ÷ 3 = 3 cm. The 3-hour mean is (1 + 2 + 3) ÷ 3 = 2 cm. The observed mean difference is 1 cm in this supplied week. Compare Means and Variation: L mean = (2 + 4 + 6)/3 = 4 cm; range 2–6. S mean = (1 + 3 + 5)/3 = 3 cm; range 1–5. L’s mean is 1 cm higher, but its smallest result is below S’s largest. Spot Two Changes at Once: A averages 5 cm and B 2 cm: a 3 cm observed difference. Both light and water changed, so these records cannot identify light alone as the cause. A repaired plan uses equal water for both groups. Plan a Replication and Report Its Scope: The repeat means are both (1 + 2 + 3)/3 = 2 cm. Unlike the seed’s 1 cm gap, this repeat has no mean gap. The two small trials give mixed results, so the claim should remain tentative.
Make a practice plan
- Try the supplied lesson problems without looking at the answers.
- Compare your method with the worked examples and explain any correction.
- Use the new review cases, then complete the transfer task on paper or in your own document.