Evidence and Precision in an Urban Trees Argument
A student is drafting an argumentative essay for a class assignment. The essay claims that planting trees is the most effective way to lower summer energy costs in cities. The student has gathered three sources. Source A is a city report showing that neighborhoods with 30% tree canopy have average July electricity bills $18 lower than neighborhoods with 10% canopy. Source B is a university study finding that shade from trees reduces surface temperatures by 5–7°F on hot days. Source C is a resident survey in which 70% of respondents said trees improve neighborhood aesthetics. The student wants to use the strongest evidence to support the claim and to avoid overstating what the data show. The draft currently includes the sentence: 'Planting trees is the most effective way to lower summer energy costs in cities.'
1. Which source best supports the claim as written, and why do the other sources fall short?
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Source A best supports the claim because it directly connects greater tree canopy (30% vs. 10%) to lower July electricity bills ($18 lower). Source B supports a mechanism-shade reduces surface temperatures-but it does not compare trees with other cooling methods, so it cannot establish that trees are the 'most effective' way. Source C addresses aesthetics, not energy costs, so it is irrelevant to the claim. Moreover, the claim uses 'most effective,' which requires a comparison with alternatives such as reflective roofs or insulation; none of the sources provides that comparison. Therefore, Source A is the strongest evidence, but even it only shows an association, not proof of causation.
2. Revise the student’s claim to align more precisely with the evidence available.
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A more precise claim would be: 'Planting trees can help lower summer energy costs in cities; neighborhoods with more tree canopy tend to have lower July electricity bills.' This revision removes the unsupported superlative 'most effective' and uses cautious language ('can help,' 'tend to') that matches the correlational evidence. It also avoids implying causation from a single city report. If the student wants to keep a stronger claim, they would need evidence comparing tree planting directly with other cooling strategies.
3. Explain why Source C is not useful for supporting the energy-cost claim.
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Source C is a survey about residents' opinions on aesthetics. It does not measure energy costs, electricity use, or temperature. Even if 70% of respondents like trees, that says nothing about whether trees lower energy bills. Using it to support an energy-cost claim would be a non sequitur. It could be used in a different paragraph about quality of life, but not for the energy argument.