Concrete testing. A company that specializes in developing concrete for construction strives to continually improve the properties of its materials. To increase the compressive 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 boxplots show the test results on compressive strength (in psi) for the three levels of alkali content and three types of sand. Twoway ANOVA results are also given.

Source DF Sum of Squares Mean Square F-ratio P-value Alkali Content 2 4016600 2008300 46.38 60.0001 Sand Type 2 1547817 773908 17.87 60.0001 Interaction 4 177533 44383 1.02 0.412 Error 27 1169250 43306 Total 35 6911200

a) Is this an experiment or an observational study? Explain.

b) Are the conditions for two-way ANOVA met?

c) If so, perform the hypothesis tests and state your conclusions in terms of compressive strength, alkali content, and sand type.

d) Is it appropriate to interpret the main effects in this case? Explain.