Fifth-Grade Fair-Test Revision
A fifth-grade class of 26 students has just finished a three-week plant-growth investigation. Each of six groups chose its own fertilizer amount, but groups also differed in window distance, watering amount, pot size, and seed type. Group A, with the most fertilizer and a south-facing window, has the tallest plants. Group B, with no fertilizer and a north-facing window, has short yellow plants. Group C, with fertilizer but overwatering, has no growth. Most students conclude, 'More fertilizer makes plants grow taller.' The teacher has one 30-minute debrief before the class moves to a new unit in two weeks. The standards require students to design fair tests, control variables, and support claims with evidence. Available materials include rapid-cycling radish seeds, same-size pots, grow lights with timers, rulers, and a shared data table. The teacher wants students to revise their explanation, not just hear the correct answer.
1. Identify the design flaw and explain how it affects the students' claim.
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The design is confounded: fertilizer amount is not the only difference among groups. Group A's height could be due to more light, not fertilizer; Group B's yellowing could be due to low light or no fertilizer; Group C's failure could be overwatering. Because at least four variables changed at once, the comparison cannot isolate the effect of fertilizer. The conclusion 'more fertilizer makes plants taller' is not supported even if it happens to be true. The teacher should name the flaw as a fair-test problem, not a student character problem, and ask which variable each group actually changed.
2. Plan a revised investigation that fits the available time and materials, and explain how it would support a fair test.
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Revise with a common protocol: same seed type, same pot size, same soil amount, same watering schedule (measured), same distance from a grow light, and fertilizer amount as the only variable. Use at least three fertilizer levels plus a control (0, low, recommended), with two or three pots per level to reduce chance effects. Randomize pot positions under the light and rotate them. Measure height, leaf color, and number of leaves every two days for 10-14 days with rapid-cycling radishes. Students write a procedure and a variables table before planting. If time is short, run the revised investigation as a whole-class demonstration with student data collectors, but keep the design intact.
3. How should the teacher assess students' revised written claims?
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Assess written claims with a claim-evidence-reasoning check: does the claim answer the fertilizer question; does the evidence come from the revised data table; does the reasoning explain why controlled variables make the comparison fair? A proficient response says, for example, 'Plants with recommended fertilizer were taller than the no-fertilizer control, and because light, water, and pot size were the same, the difference is more likely due to fertilizer.' Students should also identify one limitation, such as only two weeks or one seed type. The teacher can use a quick exit ticket with a flawed claim and ask students to revise it, which reveals whether they can apply the fair-test idea.