Duckweed, fertiliser and a sunny windowsill
Amara is investigating whether a liquid fertiliser increases duckweed growth. She places 20 duckweed fronds into each of two 400 cm³ beakers. Beaker A contains 250 cm³ pond water plus 5 drops of fertiliser; beaker B contains 250 cm³ pond water and no fertiliser. She puts beaker A on the south-facing windowsill because there is more room there, and beaker B on a shaded side bench. After 14 days she counts the fronds: A has 74, B has 31. She concludes that fertiliser roughly triples duckweed growth. Her teacher notices a green film of algae on the surface of beaker A and that each beaker was set up only once. Amara says the algae are irrelevant because she is counting duckweed, not algae.
1. Identify the main variable that is confounded with the fertiliser treatment in Amara's design, and explain how it could produce the observed result even if the fertiliser had no effect.
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The fertiliser treatment is confounded with light: beaker A is on the sunny windowsill while B is in shade. Light is a limiting factor for photosynthesis in duckweed, so A could grow faster simply because it receives more light. The algae point to a second difference: the fertiliser may have fed the algae, and the algal film could shade the duckweed or change oxygen and carbon dioxide levels. Because light and fertiliser vary together, the results cannot show which caused the extra growth. A valid conclusion needs the two treatments to differ only in the presence of fertiliser.
2. Explain two further weaknesses in the investigation and describe specific changes that would make the conclusion more trustworthy.
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Two weaknesses are: (1) there is no replication - each treatment has one beaker, so a single accident or difference cannot be separated from a treatment effect; (2) counting 'fronds' is subjective and may not measure growth, because fronds split and small fronds may be missed. The initial frond size was also assumed equal, and algae were uncontrolled. Improvements: use at least five beakers per treatment, assign positions at random and rotate them, keep all beakers under the same light and temperature, use the same batch of pond water, and remove or control algae. Measure dry mass or count fronds using a grid and a defined rule. Include a control with the same volume of water but no fertiliser, then compare mean growth and the spread of results.
3. State the conclusion that the data do support and explain why the original conclusion is not justified.
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The data support only the statement that, in these two beakers, the one with fertiliser and more light ended with more fronds (74 versus 31). They do not support 'fertiliser triples growth' because the cause is confounded and there is no replication. Even if the difference is real, one pair of beakers cannot establish reliability, and 'triples' is a precise claim based on one comparison. The justified conclusion is that the result is consistent with fertiliser increasing growth, but a controlled, replicated experiment is needed before accepting that conclusion.