Problem-Solving and Data AnalysisVery hardStrengthening a causal claim
Problem-Solving and Data Analysis practice question
A gardener reports that tomato plants she fed with a new fertiliser produced more fruit than plants she fed with the old one. Which of the following would most strengthen the conclusion that the new fertiliser caused the difference?
- A. She had assigned the two fertilisers to plants at random, kept everything else the same, and repeated the comparison over three seasons with the same result.Correct
- B. She used the new fertiliser on the plants in the sunniest bed.
- C. The new fertiliser costs more.
- D. The new fertiliser's packet promises bigger crops.
Answer: A. She had assigned the two fertilisers to plants at random, kept everything else the same, and repeated the comparison over three seasons with the same result.
Random assignment, control of other factors, and replication.
Why the other answers are wrong
- B. She used the new fertiliser on the plants in the sunniest bed.
- Sunlight would confound the comparison.
- C. The new fertiliser costs more.
- Price says nothing about effect.
- D. The new fertiliser's packet promises bigger crops.
- A packet's claim is not evidence.
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