A Fair Test That Isn't Fair
Third graders are investigating: "Do plants need light to grow?" Each group plants two bean seeds in identical cups with the same soil and water. Group A puts one cup on the sunny windowsill and one in a dark closet, but waters the sunny cup daily and the closet cup only once. Group B puts both cups on the windowsill but covers one with a box; both get the same water. Group C puts one cup in the closet and one on the windowsill and waters both the same, but uses different amounts of soil. When results come in, Group A's sunny plant is much taller, and students conclude "light makes plants grow." Ms. Rivera wants students to evaluate whether the investigation supports the conclusion.
1. Evaluate each group's setup as a fair test and explain what can and cannot be concluded.
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A fair test changes one variable (light) and holds all others (water, soil, container, seed type) constant.
- Group A: Changes light and water. Confounded-the taller sunny plant could be due to more water, not light. Cannot support the light conclusion.
- Group B: Changes only light (box vs. no box); water, soil, and location are the same. This is a fair test and can support a conclusion about light.
- Group C: Changes light and soil amount. Confounded-soil volume differs, so growth differences can't be attributed to light alone.
So the class conclusion "light makes plants grow" is not supported by A or C, only potentially by B. The class should rely on B's data and revise A and C.
2. Design a follow-up that turns this into a lesson on experimental design.
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Have students conduct a variable audit: for each group, list what was the same and what was different, then circle any setup with more than one difference. Students then redesign Group A and Group C so only light varies (same water schedule, same soil volume).
Extend with a prediction task: "If we repeat Group B three times and the boxed plant is always shorter, how confident are we?" This introduces replication and the idea that a single trial is weak evidence. Close with a written claim-evidence-reasoning paragraph: claim (light affects growth), evidence (Group B data), reasoning (only light differed). Assess on whether students cite the controlled comparison, not just the result.