Review: Variation, controls, and scientific claims
Evaluate small comparisons using means, variation, and fair design.
Bring the unit skills together
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Use the skills in a new task
Review a fictional research note: a 5-hour light group grown in soil X has growths 2, 5, 8 cm; a 2-hour group in soil Y has 3, 4, 5 cm. Calculate means and ranges, explain a design limit, and write a revised paper plan for a second trial.
Work on paper or in your own document. Explain the choices you made.
- Calculate means 5 and 4 cm and ranges 2–8 and 3–5.
- Describe the 1 cm mean gap and overlapping ranges.
- Identify soil as a confound preventing a light-only attribution.
- Plan same soil, species, start size, water, pots, temperature, duration, and random assignment with more seedlings.
Compare with a model response
The group means are 5 and 4 cm, a 1 cm gap. Their ranges are 2–8 and 3–5 cm, so they overlap. Soil differs along with light; this record cannot isolate light as the cause. My second paper plan uses more same-species seedlings, random labels for assignment, the same soil, starting size, water, pots, temperature, and duration, changing only 5 versus 2 light-hours and recording individual height growth.
A different response can also meet the criteria. Compare the evidence and reasoning, rather than copying the wording.