Choosing Evidence for a Science Report
A student investigates whether plants grow taller near a window. She places 10 identical bean seedlings in pots: 5 on a sunny windowsill and 5 in a shaded corner. All receive the same amount of water. After 3 weeks, windowsill plants average 14 cm and shaded plants average 9 cm. However, the shaded corner is also next to a heater that runs during the day, and the student did not record room temperatures. She writes: "Plants need sunlight to grow; this proves sunlight is the only factor."
1. Explain why the conclusion "sunlight is the only factor" is not supported by this investigation.
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The conclusion is not supported because the investigation did not control all variables. The shaded plants were also near a heater, so temperature differed between the two groups. The 5 cm difference could be caused by light, by heat, or by both together. Saying sunlight is the "only factor" is an overclaim: the data show an association between position and height, not that light alone caused the difference. To support that claim, the student would need to control temperature (record it, or move the heater) and keep every other condition identical. The valid conclusion is narrower: "Plants on the windowsill grew taller on average, but temperature was not controlled, so the cause is uncertain."
2. The student wants to strengthen the investigation without starting over. Suggest one change and explain how it would improve the evidence.
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A practical change is to record the temperature in both locations daily and, if possible, move the heater away from the shaded group so both groups experience similar temperatures. Reasoning: this removes the confound. If the shaded plants still grow less after temperature is matched, the light explanation becomes much stronger because an alternative cause has been ruled out. Alternatively, she could add a third group in shade with no heater to separate the effects of light and heat. Either change turns a single comparison into evidence that can support a causal claim, rather than only describing a difference.