Question: Concrete testing. A company that specializes in developing concrete for construction strives to continually improve the properties of its materials. In order to increase the comprehensive strength of one of its new formulations, they varied the amount of alkali content (low, medium, high). Since the type of sand used may also affect the strength of concrete, they used three different types of sand (Types I, II, III). Four samples were randomly selected from each treatment combination to be tested. The box- plots show the test results on compressive strength (in psi) for the three levels of alkali content and three types of sand. Two- way ANOVA results are also given.

1. Is this an experiment or an observational study? Explain.
2. Are the conditions for two-way ANOVA met?
3. If so, perform the hypothesis tests and state your conclusions in terms of compressive strength, alkali content, and sand type.
4. Is it appropriate to interpret the main effects in this case? Explain.

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