Question: The concentration of carbon dioxide (CO ₂ ) in the atmosphere is increasing rapidly due to our use of fossil fuels. Because plants use CO ₂ to fuel photosynthesis, more CO ₂ may cause trees and other plants to grow faster. An elaborate apparatus allows researchers to pipe extra CO ₂ to a 30-meter circle of forest. They selected two nearby circles in each of three parts of a pine forest and randomly chose one of each pair to receive extra CO ₂ . The response variable is the mean increase in base are for 30 to 40 trees in a circle during a growing season. We measure this in percent increase per year. Here are one year’s data:

1. State the null and alternative hypotheses. Explain clearly why the investigators used one-sided alternative.
2. Carry out a test and report your conclusion in simple language.
3. The investigators used the test you just carried out. Any use of the t-procedures with sample this size is risky, why?

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